diff --git a/README.md b/README.md
index 79e81c8..18f5987 100644
--- a/README.md
+++ b/README.md
@@ -30,9 +30,11 @@ A billing and payment provider plugin for PayloadCMS 3.x. Supports Stripe, Molli
 - 👥 Flexible customer data management with relationship support
 - 📊 Complete payment tracking and history
 - 🪝 Secure webhook processing for all providers
+- 🔄 Automatic payment/invoice status synchronization
 - 🧪 Built-in test provider for local development
 - 📱 Payment management in PayloadCMS admin
-- 🔄 Callback-based customer data syncing
+- 🔗 Bidirectional payment-invoice relationship management
+- 🎨 Collection extension support for custom fields and hooks
 - 🔒 Full TypeScript support
 
 ## Installation
@@ -197,6 +199,16 @@ The plugin adds these collections:
 - **invoices** - Invoice generation with line items and embedded customer info
 - **refunds** - Refund tracking and management
 
+### Automatic Status Synchronization
+
+The plugin automatically keeps payments and invoices in sync:
+
+- **Payment → Invoice**: When a payment status changes to `paid` or `succeeded`, any linked invoice is automatically updated to `paid` status
+- **Invoice → Payment**: When an invoice is created with a payment link, the payment is automatically linked back (bidirectional relationship)
+- **Manual Invoice Payment**: When an invoice status is manually changed to `paid`, the linked payment is updated to `succeeded`
+
+This ensures data consistency without manual intervention and works seamlessly with webhook updates from payment providers.
+
 ### Customer Data Management
 
 The plugin supports flexible customer data handling:
diff --git a/dev/README.md b/dev/README.md
index 4667eee..a739747 100644
--- a/dev/README.md
+++ b/dev/README.md
@@ -7,10 +7,13 @@ This is a demo application showcasing the `@xtr-dev/payload-billing` plugin for
 - 🧪 **Test Payment Provider** with customizable scenarios
 - 💳 **Payment Management** with full CRUD operations
 - 🧾 **Invoice Generation** with line items and tax calculation
-- 👥 **Customer Management** with relationship support
-- 🔄 **Refund Processing** and tracking
+- 🔄 **Automatic Status Sync** - payments and invoices stay in sync automatically
+- 🔗 **Bidirectional Relationships** - payment/invoice links maintained by plugin hooks
 - 🎨 **Custom Payment UI** with modern design
-- 📊 **Sample Data** for quick testing
+- 📄 **Invoice View Page** - professional printable invoice layout
+- 🔧 **Collection Extensions** - demonstrates how to extend collections with custom fields and hooks
+- 💬 **Custom Message Field** - shows hook-based data copying from payment to invoice
+- 📊 **No Customer Collection Required** - uses direct customer info fields
 
 ## Getting Started
 
@@ -57,6 +60,20 @@ This page demonstrates:
 - Real-time payment status updates
 - Test mode indicators and warnings
 
+### Invoice View Page
+View and print invoices at:
+```
+http://localhost:3000/invoice/{invoice-id}
+```
+
+This page demonstrates:
+- Professional printable invoice layout
+- Customer billing information
+- Line items table with quantities and amounts
+- Tax calculations and totals
+- Custom message field (populated from payment metadata)
+- Print-friendly styling
+
 ### Admin Routes
 
 - **Payments**: [http://localhost:3000/admin/collections/payments](http://localhost:3000/admin/collections/payments)
@@ -111,6 +128,57 @@ testProvider({
 })
 ```
 
+### Collection Extension Options
+
+This demo showcases how to extend the plugin's collections with custom fields and hooks. The invoices collection is extended to include a `customMessage` field that is automatically populated from payment metadata:
+
+```typescript
+collections: {
+  payments: 'payments',
+  invoices: {
+    slug: 'invoices',
+    extend: (config) => ({
+      ...config,
+      fields: [
+        ...(config.fields || []),
+        {
+          name: 'customMessage',
+          type: 'textarea',
+          admin: {
+            description: 'Custom message from the payment (auto-populated)',
+          },
+        },
+      ],
+      hooks: {
+        ...config.hooks,
+        beforeChange: [
+          ...(config.hooks?.beforeChange || []),
+          async ({ data, req, operation }) => {
+            if (operation === 'create' && data.payment) {
+              const payment = await req.payload.findByID({
+                collection: 'payments',
+                id: typeof data.payment === 'object' ? data.payment.id : data.payment,
+              })
+
+              if (
+                payment?.metadata &&
+                typeof payment.metadata === 'object' &&
+                'customMessage' in payment.metadata &&
+                payment.metadata.customMessage
+              ) {
+                data.customMessage = payment.metadata.customMessage as string
+              }
+            }
+            return data
+          },
+        ],
+      },
+    }),
+  },
+  refunds: 'refunds',
+}
+```
+
 ### Customer Relationship
 ```typescript
 customerRelationSlug: 'customers',
@@ -228,10 +296,13 @@ Request body:
 {
   "amount": 2500,
   "currency": "USD",
-  "description": "Demo payment"
+  "description": "Demo payment",
+  "message": "Custom message to include in the invoice (optional)"
 }
 ```
 
+The `message` field will be stored in the payment's metadata and automatically copied to the invoice when it's created, thanks to the collection extension hook.
+
 Response:
 ```json
 {
@@ -246,6 +317,68 @@ Response:
 }
 ```
 
+### Get Payment
+```
+GET /api/demo/payment/{payment-provider-id}
+```
+
+Fetches payment details including invoice relationship. Used by the payment success page to find the associated invoice.
+
+Response:
+```json
+{
+  "success": true,
+  "payment": {
+    "id": "67890",
+    "providerId": "test_pay_1234567890_abc123",
+    "amount": 2500,
+    "currency": "USD",
+    "status": "paid",
+    "description": "Demo payment",
+    "invoice": "invoice-id-here",
+    "metadata": {
+      "customMessage": "Your custom message"
+    }
+  }
+}
+```
+
+### Get Invoice
+```
+GET /api/demo/invoice/{invoice-id}
+```
+
+Fetches complete invoice data including customer details, line items, and custom message. Used by the invoice view page.
+
+Response:
+```json
+{
+  "success": true,
+  "invoice": {
+    "id": "invoice-id",
+    "invoiceNumber": "INV-2024-001",
+    "customer": {
+      "name": "John Doe",
+      "email": "john@example.com",
+      "company": "Acme Corp"
+    },
+    "currency": "USD",
+    "items": [
+      {
+        "description": "Service",
+        "quantity": 1,
+        "unitAmount": 2500
+      }
+    ],
+    "subtotal": 2500,
+    "taxAmount": 250,
+    "total": 2750,
+    "status": "paid",
+    "customMessage": "Your custom message from payment"
+  }
+}
+```
+
 ## Development
 
 ### File Structure
@@ -257,10 +390,23 @@ dev/
 │   ├── test-payment/
 │   │   └── [id]/
 │   │       └── page.tsx             # Custom payment UI
+│   ├── invoice/
+│   │   └── [id]/
+│   │       └── page.tsx             # Invoice view/print page
+│   ├── payment-success/
+│   │   └── page.tsx                 # Payment success page
+│   ├── payment-failed/
+│   │   └── page.tsx                 # Payment failed page
 │   ├── api/
 │   │   └── demo/
-│   │       └── create-payment/
-│   │           └── route.ts         # Payment creation endpoint
+│   │       ├── create-payment/
+│   │       │   └── route.ts         # Payment creation endpoint
+│   │       ├── invoice/
+│   │       │   └── [id]/
+│   │       │       └── route.ts     # Invoice fetch endpoint
+│   │       └── payment/
+│   │           └── [id]/
+│   │               └── route.ts     # Payment fetch endpoint
 │   └── (payload)/                   # PayloadCMS admin routes
 ├── helpers/
 │   └── credentials.ts               # Default user credentials
diff --git a/dev/app/api/demo/create-payment/route.ts b/dev/app/api/demo/create-payment/route.ts
index 308eec0..ff5d5c8 100644
--- a/dev/app/api/demo/create-payment/route.ts
+++ b/dev/app/api/demo/create-payment/route.ts
@@ -8,7 +8,10 @@ export async function POST(request: Request) {
     })
 
     const body = await request.json()
-    const { amount, currency, description } = body
+    const { amount, currency, description, message, customerName, customerEmail, customerCompany } = body
+
+    // eslint-disable-next-line no-console
+    console.log('Received payment request:', { amount, currency, customerName, customerEmail, customerCompany })
 
     if (!amount || !currency) {
       return Response.json(
@@ -17,7 +20,16 @@ export async function POST(request: Request) {
       )
     }
 
-    // Create a payment using the test provider
+    if (!customerName || !customerEmail) {
+      // eslint-disable-next-line no-console
+      console.log('Missing customer info:', { customerName, customerEmail })
+      return Response.json(
+        { success: false, error: 'Customer name and email are required' },
+        { status: 400 }
+      )
+    }
+
+    // Create a payment first using the test provider
     const payment = await payload.create({
       collection: 'payments',
       data: {
@@ -29,10 +41,42 @@ export async function POST(request: Request) {
         metadata: {
           source: 'demo-ui',
           createdAt: new Date().toISOString(),
+          customMessage: message, // Store the custom message in metadata
         },
       },
     })
 
+    // Create an invoice linked to the payment
+    // The invoice's afterChange hook will automatically link the payment back to the invoice
+    const invoice = await payload.create({
+      collection: 'invoices',
+      data: {
+        payment: payment.id, // Link to the payment
+        customerInfo: {
+          name: customerName,
+          email: customerEmail,
+          company: customerCompany,
+        },
+        billingAddress: {
+          line1: '123 Demo Street',
+          city: 'Demo City',
+          state: 'DC',
+          postalCode: '12345',
+          country: 'US',
+        },
+        currency,
+        items: [
+          {
+            description: description || 'Demo payment',
+            quantity: 1,
+            unitAmount: amount,
+          },
+        ],
+        taxAmount: 0,
+        status: 'open',
+      },
+    })
+
     return Response.json({
       success: true,
       payment: {
@@ -41,6 +85,7 @@ export async function POST(request: Request) {
         amount: payment.amount,
         currency: payment.currency,
         description: payment.description,
+        invoiceId: invoice.id,
       },
     })
   } catch (error) {
diff --git a/dev/app/api/demo/invoice/[id]/route.ts b/dev/app/api/demo/invoice/[id]/route.ts
new file mode 100644
index 0000000..92fd64a
--- /dev/null
+++ b/dev/app/api/demo/invoice/[id]/route.ts
@@ -0,0 +1,116 @@
+import configPromise from '@payload-config'
+import { getPayload } from 'payload'
+
+export async function GET(request: Request, { params }: { params: Promise<{ id: string }> }) {
+  try {
+    const payload = await getPayload({
+      config: configPromise,
+    })
+
+    const { id: invoiceId } = await params
+
+    if (!invoiceId) {
+      return Response.json(
+        { success: false, error: 'Invoice ID is required' },
+        { status: 400 }
+      )
+    }
+
+    // Fetch the invoice
+    const invoice = await payload.findByID({
+      collection: 'invoices',
+      id: invoiceId,
+    })
+
+    if (!invoice) {
+      return Response.json(
+        { success: false, error: 'Invoice not found' },
+        { status: 404 }
+      )
+    }
+
+    // Get customer info - either from relationship or direct fields
+    let customerInfo = null
+
+    if (invoice.customer) {
+      // Try to fetch from customer relationship
+      try {
+        const customerData = await payload.findByID({
+          collection: 'customers',
+          id: typeof invoice.customer === 'object' ? invoice.customer.id : invoice.customer,
+        })
+        customerInfo = {
+          name: customerData.name,
+          email: customerData.email,
+          phone: customerData.phone,
+          company: customerData.company,
+          taxId: customerData.taxId,
+          billingAddress: customerData.address,
+        }
+      } catch (error) {
+        // Customer not found or collection doesn't exist
+        console.error('Failed to fetch customer:', error)
+      }
+    }
+
+    // Fall back to direct customerInfo fields if no customer relationship
+    if (!customerInfo && invoice.customerInfo) {
+      customerInfo = {
+        name: invoice.customerInfo.name,
+        email: invoice.customerInfo.email,
+        phone: invoice.customerInfo.phone,
+        company: invoice.customerInfo.company,
+        taxId: invoice.customerInfo.taxId,
+        billingAddress: invoice.billingAddress,
+      }
+    }
+
+    // Default customer if neither is available
+    if (!customerInfo) {
+      customerInfo = {
+        name: 'Unknown Customer',
+        email: 'unknown@example.com',
+      }
+    }
+
+    // Calculate subtotal from items (or use stored subtotal)
+    const subtotal = invoice.subtotal || invoice.items?.reduce((sum: number, item: any) => {
+      return sum + (item.unitAmount * item.quantity)
+    }, 0) || 0
+
+    const taxAmount = invoice.taxAmount || 0
+    const total = invoice.amount || (subtotal + taxAmount)
+
+    // Prepare the response
+    const invoiceData = {
+      id: invoice.id,
+      invoiceNumber: invoice.number || invoice.invoiceNumber,
+      customer: customerInfo,
+      currency: invoice.currency,
+      items: invoice.items || [],
+      subtotal,
+      taxAmount,
+      total,
+      status: invoice.status,
+      customMessage: invoice.customMessage,
+      issuedAt: invoice.issuedAt,
+      dueDate: invoice.dueDate,
+      createdAt: invoice.createdAt,
+    }
+
+    return Response.json({
+      success: true,
+      invoice: invoiceData,
+    })
+  } catch (error) {
+    // eslint-disable-next-line no-console
+    console.error('Failed to fetch invoice:', error)
+    return Response.json(
+      {
+        success: false,
+        error: error instanceof Error ? error.message : 'Failed to fetch invoice',
+      },
+      { status: 500 }
+    )
+  }
+}
diff --git a/dev/app/api/demo/payment/[id]/route.ts b/dev/app/api/demo/payment/[id]/route.ts
new file mode 100644
index 0000000..7f7683c
--- /dev/null
+++ b/dev/app/api/demo/payment/[id]/route.ts
@@ -0,0 +1,63 @@
+import configPromise from '@payload-config'
+import { getPayload } from 'payload'
+
+export async function GET(request: Request, { params }: { params: Promise<{ id: string }> }) {
+  try {
+    const payload = await getPayload({
+      config: configPromise,
+    })
+
+    const { id: paymentProviderId } = await params
+
+    if (!paymentProviderId) {
+      return Response.json(
+        { success: false, error: 'Payment ID is required' },
+        { status: 400 }
+      )
+    }
+
+    // Find payment by providerId (the test provider uses this format)
+    const payments = await payload.find({
+      collection: 'payments',
+      where: {
+        providerId: {
+          equals: paymentProviderId,
+        },
+      },
+      limit: 1,
+    })
+
+    if (!payments.docs.length) {
+      return Response.json(
+        { success: false, error: 'Payment not found' },
+        { status: 404 }
+      )
+    }
+
+    const payment = payments.docs[0]
+
+    return Response.json({
+      success: true,
+      payment: {
+        id: payment.id,
+        providerId: payment.providerId,
+        amount: payment.amount,
+        currency: payment.currency,
+        status: payment.status,
+        description: payment.description,
+        invoice: payment.invoice,
+        metadata: payment.metadata,
+      },
+    })
+  } catch (error) {
+    // eslint-disable-next-line no-console
+    console.error('Failed to fetch payment:', error)
+    return Response.json(
+      {
+        success: false,
+        error: error instanceof Error ? error.message : 'Failed to fetch payment',
+      },
+      { status: 500 }
+    )
+  }
+}
diff --git a/dev/app/invoice/[id]/page.tsx b/dev/app/invoice/[id]/page.tsx
new file mode 100644
index 0000000..a59a25e
--- /dev/null
+++ b/dev/app/invoice/[id]/page.tsx
@@ -0,0 +1,317 @@
+'use client'
+
+import { useParams } from 'next/navigation'
+import { useEffect, useState } from 'react'
+
+interface InvoiceItem {
+  description: string
+  quantity: number
+  unitAmount: number
+  id?: string
+}
+
+interface Customer {
+  name: string
+  email: string
+  phone?: string
+  company?: string
+  taxId?: string
+  billingAddress?: {
+    line1: string
+    line2?: string
+    city: string
+    state?: string
+    postalCode: string
+    country: string
+  }
+}
+
+interface Invoice {
+  id: string
+  invoiceNumber: string
+  customer: Customer
+  currency: string
+  items: InvoiceItem[]
+  subtotal: number
+  taxAmount?: number
+  total: number
+  status: string
+  customMessage?: string
+  issuedAt?: string
+  dueDate?: string
+  createdAt: string
+}
+
+export default function InvoiceViewPage() {
+  const params = useParams()
+  const invoiceId = params.id as string
+  const [invoice, setInvoice] = useState<Invoice | null>(null)
+  const [loading, setLoading] = useState(true)
+  const [error, setError] = useState<string>('')
+
+  useEffect(() => {
+    fetchInvoice()
+  }, [invoiceId])
+
+  const fetchInvoice = async () => {
+    try {
+      const response = await fetch(`/api/demo/invoice/${invoiceId}`)
+      const data = await response.json()
+
+      if (data.success) {
+        setInvoice(data.invoice)
+      } else {
+        setError(data.error || 'Failed to load invoice')
+      }
+    } catch (err) {
+      setError(err instanceof Error ? err.message : 'An error occurred')
+    } finally {
+      setLoading(false)
+    }
+  }
+
+  const handlePrint = () => {
+    window.print()
+  }
+
+  if (loading) {
+    return (
+      <div className="min-h-screen bg-slate-50 flex items-center justify-center p-4">
+        <div className="text-slate-600 text-lg">Loading invoice...</div>
+      </div>
+    )
+  }
+
+  if (error || !invoice) {
+    return (
+      <div className="min-h-screen bg-slate-50 flex items-center justify-center p-4">
+        <div className="max-w-2xl w-full bg-white rounded-lg shadow-lg p-8">
+          <div className="text-center">
+            <div className="text-red-600 text-5xl mb-4">⚠️</div>
+            <h1 className="text-2xl font-bold text-slate-800 mb-2">Invoice Not Found</h1>
+            <p className="text-slate-600 mb-6">{error || 'The requested invoice could not be found.'}</p>
+            <a
+              href="/"
+              className="inline-block bg-blue-600 text-white px-6 py-3 rounded-lg font-semibold hover:bg-blue-700 transition-colors"
+            >
+              Back to Demo
+            </a>
+          </div>
+        </div>
+      </div>
+    )
+  }
+
+  const formatCurrency = (amount: number) => {
+    return `${invoice.currency.toUpperCase()} ${(amount / 100).toFixed(2)}`
+  }
+
+  const formatDate = (dateString: string) => {
+    return new Date(dateString).toLocaleDateString('en-US', {
+      year: 'numeric',
+      month: 'long',
+      day: 'numeric',
+    })
+  }
+
+  return (
+    <div className="min-h-screen bg-slate-50 py-8 print:bg-white print:py-0">
+      <div className="max-w-4xl mx-auto px-4">
+        {/* Print Button - Hidden when printing */}
+        <div className="mb-6 flex justify-end print:hidden">
+          <button
+            onClick={handlePrint}
+            className="bg-blue-600 text-white px-6 py-3 rounded-lg font-semibold hover:bg-blue-700 transition-colors flex items-center gap-2"
+          >
+            <svg className="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+              <path
+                strokeLinecap="round"
+                strokeLinejoin="round"
+                strokeWidth={2}
+                d="M17 17h2a2 2 0 002-2v-4a2 2 0 00-2-2H5a2 2 0 00-2 2v4a2 2 0 002 2h2m2 4h6a2 2 0 002-2v-4a2 2 0 00-2-2H9a2 2 0 00-2 2v4a2 2 0 002 2zm8-12V5a2 2 0 00-2-2H9a2 2 0 00-2 2v4h10z"
+              />
+            </svg>
+            Print Invoice
+          </button>
+        </div>
+
+        {/* Invoice Container */}
+        <div className="bg-white rounded-lg shadow-lg print:shadow-none print:rounded-none">
+          <div className="p-8 md:p-12">
+            {/* Header */}
+            <div className="mb-8 pb-8 border-b-2 border-slate-200">
+              <div className="flex justify-between items-start mb-6">
+                <div>
+                  <h1 className="text-4xl font-bold text-slate-800 mb-2">INVOICE</h1>
+                  <p className="text-slate-600">Invoice #{invoice.invoiceNumber}</p>
+                </div>
+                <div className="text-right">
+                  <div className="text-2xl font-bold text-blue-600 mb-1">
+                    @xtr-dev/payload-billing
+                  </div>
+                  <p className="text-slate-600 text-sm">Test Provider Demo</p>
+                </div>
+              </div>
+
+              <div className="grid md:grid-cols-2 gap-8">
+                {/* Bill To */}
+                <div>
+                  <h2 className="text-sm font-semibold text-slate-500 uppercase mb-3">Bill To</h2>
+                  <div className="text-slate-800">
+                    <p className="font-semibold text-lg">{invoice.customer.name}</p>
+                    {invoice.customer.company && (
+                      <p className="text-slate-600">{invoice.customer.company}</p>
+                    )}
+                    <p className="text-slate-600">{invoice.customer.email}</p>
+                    {invoice.customer.phone && (
+                      <p className="text-slate-600">{invoice.customer.phone}</p>
+                    )}
+                    {invoice.customer.billingAddress && (
+                      <div className="mt-2 text-slate-600">
+                        <p>{invoice.customer.billingAddress.line1}</p>
+                        {invoice.customer.billingAddress.line2 && (
+                          <p>{invoice.customer.billingAddress.line2}</p>
+                        )}
+                        <p>
+                          {invoice.customer.billingAddress.city}
+                          {invoice.customer.billingAddress.state && `, ${invoice.customer.billingAddress.state}`} {invoice.customer.billingAddress.postalCode}
+                        </p>
+                        <p>{invoice.customer.billingAddress.country}</p>
+                      </div>
+                    )}
+                    {invoice.customer.taxId && (
+                      <p className="mt-2 text-slate-600">Tax ID: {invoice.customer.taxId}</p>
+                    )}
+                  </div>
+                </div>
+
+                {/* Invoice Details */}
+                <div className="text-right md:text-left">
+                  <h2 className="text-sm font-semibold text-slate-500 uppercase mb-3">Invoice Details</h2>
+                  <div className="space-y-2 text-slate-800">
+                    <div className="flex justify-between md:justify-start md:gap-4">
+                      <span className="text-slate-600">Status:</span>
+                      <span
+                        className={`px-3 py-1 rounded-full text-xs font-semibold ${
+                          invoice.status === 'paid'
+                            ? 'bg-green-100 text-green-800'
+                            : invoice.status === 'open'
+                              ? 'bg-blue-100 text-blue-800'
+                              : invoice.status === 'void'
+                                ? 'bg-red-100 text-red-800'
+                                : 'bg-slate-100 text-slate-800'
+                        }`}
+                      >
+                        {invoice.status.toUpperCase()}
+                      </span>
+                    </div>
+                    <div className="flex justify-between md:justify-start md:gap-4">
+                      <span className="text-slate-600">Issued:</span>
+                      <span className="font-medium">
+                        {formatDate(invoice.issuedAt || invoice.createdAt)}
+                      </span>
+                    </div>
+                    {invoice.dueDate && (
+                      <div className="flex justify-between md:justify-start md:gap-4">
+                        <span className="text-slate-600">Due:</span>
+                        <span className="font-medium">{formatDate(invoice.dueDate)}</span>
+                      </div>
+                    )}
+                  </div>
+                </div>
+              </div>
+            </div>
+
+            {/* Custom Message */}
+            {invoice.customMessage && (
+              <div className="mb-8 p-4 bg-blue-50 border border-blue-200 rounded-lg">
+                <h3 className="text-sm font-semibold text-blue-900 uppercase mb-2">Message</h3>
+                <p className="text-blue-800 whitespace-pre-wrap">{invoice.customMessage}</p>
+              </div>
+            )}
+
+            {/* Line Items Table */}
+            <div className="mb-8">
+              <table className="w-full">
+                <thead>
+                  <tr className="border-b-2 border-slate-300">
+                    <th className="text-left py-3 text-slate-700 font-semibold">Description</th>
+                    <th className="text-right py-3 text-slate-700 font-semibold w-24">Qty</th>
+                    <th className="text-right py-3 text-slate-700 font-semibold w-32">Unit Price</th>
+                    <th className="text-right py-3 text-slate-700 font-semibold w-32">Amount</th>
+                  </tr>
+                </thead>
+                <tbody>
+                  {invoice.items.map((item, index) => (
+                    <tr key={item.id || index} className="border-b border-slate-200">
+                      <td className="py-4 text-slate-800">{item.description}</td>
+                      <td className="py-4 text-right text-slate-800">{item.quantity}</td>
+                      <td className="py-4 text-right text-slate-800">
+                        {formatCurrency(item.unitAmount)}
+                      </td>
+                      <td className="py-4 text-right text-slate-800 font-medium">
+                        {formatCurrency(item.unitAmount * item.quantity)}
+                      </td>
+                    </tr>
+                  ))}
+                </tbody>
+              </table>
+            </div>
+
+            {/* Totals */}
+            <div className="flex justify-end mb-8">
+              <div className="w-full md:w-80">
+                <div className="space-y-3">
+                  <div className="flex justify-between py-2 text-slate-700">
+                    <span>Subtotal:</span>
+                    <span className="font-medium">{formatCurrency(invoice.subtotal)}</span>
+                  </div>
+                  {invoice.taxAmount !== undefined && invoice.taxAmount > 0 && (
+                    <div className="flex justify-between py-2 text-slate-700">
+                      <span>Tax:</span>
+                      <span className="font-medium">{formatCurrency(invoice.taxAmount)}</span>
+                    </div>
+                  )}
+                  <div className="flex justify-between py-3 border-t-2 border-slate-300 text-lg font-bold text-slate-900">
+                    <span>Total:</span>
+                    <span>{formatCurrency(invoice.total)}</span>
+                  </div>
+                </div>
+              </div>
+            </div>
+
+            {/* Footer */}
+            <div className="pt-8 border-t border-slate-200 text-center text-slate-500 text-sm">
+              <p>Thank you for your business!</p>
+              <p className="mt-2">
+                This is a demo invoice generated by @xtr-dev/payload-billing plugin
+              </p>
+            </div>
+          </div>
+        </div>
+
+        {/* Back Button - Hidden when printing */}
+        <div className="mt-6 text-center print:hidden">
+          <a
+            href="/"
+            className="inline-block text-blue-600 hover:text-blue-700 font-semibold transition-colors"
+          >
+            ← Back to Demo
+          </a>
+        </div>
+      </div>
+
+      {/* Print Styles */}
+      <style jsx global>{`
+        @media print {
+          body {
+            background: white !important;
+          }
+          @page {
+            margin: 1cm;
+          }
+        }
+      `}</style>
+    </div>
+  )
+}
diff --git a/dev/app/page.tsx b/dev/app/page.tsx
index 1d208ff..5b99375 100644
--- a/dev/app/page.tsx
+++ b/dev/app/page.tsx
@@ -7,20 +7,39 @@ export default function HomePage() {
   const [paymentId, setPaymentId] = useState<string>('')
   const [loading, setLoading] = useState(false)
   const [error, setError] = useState<string>('')
+  const [customerName, setCustomerName] = useState<string>('Demo Customer')
+  const [customerEmail, setCustomerEmail] = useState<string>('demo@example.com')
+  const [customerCompany, setCustomerCompany] = useState<string>('Demo Company')
+  const [message, setMessage] = useState<string>('')
 
   const createDemoPayment = async () => {
     setLoading(true)
     setError('')
 
+    // Validate required fields
+    if (!customerName || !customerEmail) {
+      setError('Customer name and email are required')
+      setLoading(false)
+      return
+    }
+
     try {
+      const requestBody = {
+        amount: 2500,
+        currency: 'USD',
+        description: 'Demo payment from custom UI',
+        customerName,
+        customerEmail,
+        customerCompany: customerCompany || undefined,
+        message: message || undefined,
+      }
+
+      console.log('Sending payment request:', requestBody)
+
       const response = await fetch('/api/demo/create-payment', {
         method: 'POST',
         headers: { 'Content-Type': 'application/json' },
-        body: JSON.stringify({
-          amount: 2500,
-          currency: 'USD',
-          description: 'Demo payment from custom UI',
-        }),
+        body: JSON.stringify(requestBody),
       })
 
       const data = await response.json()
@@ -64,17 +83,80 @@ export default function HomePage() {
                 </h3>
 
                 {!paymentId ? (
-                  <div>
+                  <div className="space-y-4">
+                    <div className="grid grid-cols-1 md:grid-cols-2 gap-4">
+                      <div>
+                        <label htmlFor="customerName" className="block text-sm font-medium text-slate-700 mb-2">
+                          Customer Name <span className="text-red-500">*</span>
+                        </label>
+                        <input
+                          type="text"
+                          id="customerName"
+                          value={customerName}
+                          onChange={(e) => setCustomerName(e.target.value)}
+                          placeholder="John Doe"
+                          className="w-full px-4 py-2 border border-slate-300 rounded-lg focus:ring-2 focus:ring-blue-500 focus:border-blue-500"
+                          required
+                        />
+                      </div>
+
+                      <div>
+                        <label htmlFor="customerEmail" className="block text-sm font-medium text-slate-700 mb-2">
+                          Customer Email <span className="text-red-500">*</span>
+                        </label>
+                        <input
+                          type="email"
+                          id="customerEmail"
+                          value={customerEmail}
+                          onChange={(e) => setCustomerEmail(e.target.value)}
+                          placeholder="john@example.com"
+                          className="w-full px-4 py-2 border border-slate-300 rounded-lg focus:ring-2 focus:ring-blue-500 focus:border-blue-500"
+                          required
+                        />
+                      </div>
+                    </div>
+
+                    <div>
+                      <label htmlFor="customerCompany" className="block text-sm font-medium text-slate-700 mb-2">
+                        Company Name (Optional)
+                      </label>
+                      <input
+                        type="text"
+                        id="customerCompany"
+                        value={customerCompany}
+                        onChange={(e) => setCustomerCompany(e.target.value)}
+                        placeholder="Acme Corporation"
+                        className="w-full px-4 py-2 border border-slate-300 rounded-lg focus:ring-2 focus:ring-blue-500 focus:border-blue-500"
+                      />
+                    </div>
+
+                    <div>
+                      <label htmlFor="message" className="block text-sm font-medium text-slate-700 mb-2">
+                        Custom Message (Optional)
+                      </label>
+                      <textarea
+                        id="message"
+                        rows={3}
+                        value={message}
+                        onChange={(e) => setMessage(e.target.value)}
+                        placeholder="Enter a message to include in the invoice..."
+                        className="w-full px-4 py-2 border border-slate-300 rounded-lg focus:ring-2 focus:ring-blue-500 focus:border-blue-500 resize-none"
+                      />
+                      <p className="mt-1 text-xs text-slate-500">
+                        This message will be added to the invoice using collection extension options
+                      </p>
+                    </div>
+
                     <button
                       onClick={createDemoPayment}
                       disabled={loading}
-                      className="bg-gradient-to-r from-blue-600 to-blue-700 text-white px-6 py-3 rounded-lg font-semibold hover:shadow-lg transition-all disabled:opacity-50 disabled:cursor-not-allowed cursor-pointer"
+                      className="w-full bg-gradient-to-r from-blue-600 to-blue-700 text-white px-6 py-3 rounded-lg font-semibold hover:shadow-lg transition-all disabled:opacity-50 disabled:cursor-not-allowed cursor-pointer"
                     >
                       {loading ? 'Creating Payment...' : 'Create Demo Payment'}
                     </button>
 
                     {error && (
-                      <div className="mt-4 p-4 bg-red-50 border border-red-200 text-red-800 rounded-lg">
+                      <div className="p-4 bg-red-50 border border-red-200 text-red-800 rounded-lg">
                         {error}
                       </div>
                     )}
@@ -102,6 +184,10 @@ export default function HomePage() {
                         onClick={() => {
                           setPaymentId('')
                           setError('')
+                          setCustomerName('Demo Customer')
+                          setCustomerEmail('demo@example.com')
+                          setCustomerCompany('Demo Company')
+                          setMessage('')
                         }}
                         className="bg-slate-200 text-slate-700 px-6 py-3 rounded-lg font-semibold hover:bg-slate-300 transition-all cursor-pointer"
                       >
diff --git a/dev/app/payment-success/page.tsx b/dev/app/payment-success/page.tsx
index 6d3da75..88fdc61 100644
--- a/dev/app/payment-success/page.tsx
+++ b/dev/app/payment-success/page.tsx
@@ -2,13 +2,31 @@
 
 import Link from 'next/link'
 import { useSearchParams } from 'next/navigation'
-import { Suspense } from 'react'
+import { Suspense, useEffect, useState } from 'react'
 
 function PaymentSuccessContent() {
   const searchParams = useSearchParams()
   const paymentId = searchParams.get('paymentId')
   const amount = searchParams.get('amount')
   const currency = searchParams.get('currency')
+  const [invoiceId, setInvoiceId] = useState<string | null>(null)
+
+  useEffect(() => {
+    // Fetch the payment to get the invoice ID
+    if (paymentId) {
+      fetch(`/api/demo/payment/${paymentId}`)
+        .then((res) => res.json())
+        .then((data) => {
+          if (data.success && data.payment?.invoice) {
+            const invId = typeof data.payment.invoice === 'object' ? data.payment.invoice.id : data.payment.invoice
+            setInvoiceId(invId)
+          }
+        })
+        .catch((err) => {
+          console.error('Failed to fetch payment invoice:', err)
+        })
+    }
+  }, [paymentId])
 
   return (
     <div className="min-h-screen bg-gradient-to-br from-green-600 to-emerald-700 flex items-center justify-center p-4">
@@ -76,6 +94,35 @@ function PaymentSuccessContent() {
             <h3 className="font-semibold text-slate-800 text-lg">What's Next?</h3>
 
             <div className="grid gap-3">
+              {invoiceId && (
+                <Link
+                  href={`/invoice/${invoiceId}`}
+                  className="flex items-center justify-between p-4 border-2 border-green-500 bg-green-50 rounded-lg hover:bg-green-100 transition-all group cursor-pointer"
+                >
+                  <div>
+                    <div className="font-semibold text-green-800 group-hover:text-green-900">
+                      📄 View Invoice
+                    </div>
+                    <div className="text-sm text-green-700">
+                      See your invoice with custom message
+                    </div>
+                  </div>
+                  <svg
+                    className="w-5 h-5 text-green-600 group-hover:text-green-800"
+                    fill="none"
+                    stroke="currentColor"
+                    viewBox="0 0 24 24"
+                  >
+                    <path
+                      strokeLinecap="round"
+                      strokeLinejoin="round"
+                      strokeWidth={2}
+                      d="M9 5l7 7-7 7"
+                    />
+                  </svg>
+                </Link>
+              )}
+
               <Link
                 href="/"
                 className="flex items-center justify-between p-4 border-2 border-slate-200 rounded-lg hover:border-green-500 hover:bg-green-50 transition-all group cursor-pointer"
diff --git a/dev/payload.config.ts b/dev/payload.config.ts
index 54d60d1..cf12e93 100644
--- a/dev/payload.config.ts
+++ b/dev/payload.config.ts
@@ -36,71 +36,7 @@ const buildConfigWithSQLite = () => {
           staticDir: path.resolve(dirname, 'media'),
         },
       },
-      {
-        slug: 'customers',
-        admin: {
-          useAsTitle: 'name',
-        },
-        fields: [
-          {
-            name: 'name',
-            type: 'text',
-            required: true,
-          },
-          {
-            name: 'email',
-            type: 'email',
-            required: true,
-          },
-          {
-            name: 'phone',
-            type: 'text',
-          },
-          {
-            name: 'company',
-            type: 'text',
-          },
-          {
-            name: 'taxId',
-            type: 'text',
-            label: 'Tax ID',
-          },
-          {
-            name: 'address',
-            type: 'group',
-            fields: [
-              {
-                name: 'line1',
-                type: 'text',
-                label: 'Address Line 1',
-              },
-              {
-                name: 'line2',
-                type: 'text',
-                label: 'Address Line 2',
-              },
-              {
-                name: 'city',
-                type: 'text',
-              },
-              {
-                name: 'state',
-                type: 'text',
-                label: 'State/Province',
-              },
-              {
-                name: 'postalCode',
-                type: 'text',
-                label: 'Postal Code',
-              },
-              {
-                name: 'country',
-                type: 'text',
-              },
-            ],
-          },
-        ],
-      },
+      // Note: No customers collection - the demo uses direct customerInfo fields on invoices
     ],
     db: sqliteAdapter({
       client: {
@@ -127,25 +63,62 @@ const buildConfigWithSQLite = () => {
         ],
         collections: {
           payments: 'payments',
-          invoices: 'invoices',
+          invoices: {
+            slug: 'invoices',
+            // Use extend to add custom fields and hooks to the invoice collection
+            extend: (config) => ({
+              ...config,
+              fields: [
+                ...(config.fields || []),
+                // Add a custom message field to invoices
+                {
+                  name: 'customMessage',
+                  type: 'textarea',
+                  admin: {
+                    description: 'Custom message from the payment (auto-populated)',
+                  },
+                },
+              ],
+              hooks: {
+                ...config.hooks,
+                beforeChange: [
+                  ...(config.hooks?.beforeChange || []),
+                  // Hook to copy the message from payment metadata to invoice
+                  async ({ data, req, operation }) => {
+                    // Only run on create operations
+                    if (operation === 'create' && data.payment) {
+                      try {
+                        // Fetch the related payment
+                        const payment = await req.payload.findByID({
+                          collection: 'payments',
+                          id: typeof data.payment === 'object' ? data.payment.id : data.payment,
+                        })
+
+                        // Copy the custom message from payment metadata to invoice
+                        if (
+                          payment?.metadata &&
+                          typeof payment.metadata === 'object' &&
+                          'customMessage' in payment.metadata &&
+                          payment.metadata.customMessage
+                        ) {
+                          data.customMessage = payment.metadata.customMessage as string
+                        }
+                      } catch (error) {
+                        // Log error but don't fail the invoice creation
+                        req.payload.logger.error('Failed to copy custom message to invoice:', error)
+                      }
+                    }
+                    return data
+                  },
+                ],
+              },
+            }),
+          },
           refunds: 'refunds',
         },
-        customerRelationSlug: 'customers',
-        customerInfoExtractor: (customer) => ({
-          name: customer.name,
-          email: customer.email,
-          phone: customer.phone,
-          company: customer.company,
-          taxId: customer.taxId,
-          billingAddress: customer.address ? {
-            line1: customer.address.line1,
-            line2: customer.address.line2,
-            city: customer.address.city,
-            state: customer.address.state,
-            postalCode: customer.address.postalCode,
-            country: customer.address.country,
-          } : undefined,
-        }),
+        // Note: No customerRelationSlug or customerInfoExtractor configured
+        // This allows the demo to work without a customer collection
+        // Invoices will use the direct customerInfo and billingAddress fields
       }),
     ],
     secret: process.env.PAYLOAD_SECRET || 'test-secret_key',
diff --git a/dev/seed.ts b/dev/seed.ts
index 7c39db6..1515cbb 100644
--- a/dev/seed.ts
+++ b/dev/seed.ts
@@ -83,6 +83,8 @@ async function seedBillingData(payload: Payload): Promise<void> {
       },
     })
     customer2Id = customer2.id
+  } else {
+    payload.logger.info('No customers collection found, will use direct customer info in invoices')
   }
 
   // Seed invoices
diff --git a/src/collections/invoices.ts b/src/collections/invoices.ts
index edc154d..6e05045 100644
--- a/src/collections/invoices.ts
+++ b/src/collections/invoices.ts
@@ -3,7 +3,9 @@ import type {
   CollectionAfterChangeHook,
   CollectionBeforeChangeHook,
   CollectionBeforeValidateHook,
-  CollectionConfig, Field,
+  CollectionConfig,
+  CollectionSlug,
+  Field,
 } from 'payload'
 import type { BillingPluginConfig} from '@/plugin/config';
 import { defaults } from '@/plugin/config'
@@ -13,8 +15,12 @@ import type { Invoice } from '@/plugin/types'
 
 export function createInvoicesCollection(pluginConfig: BillingPluginConfig): CollectionConfig {
   const {customerRelationSlug, customerInfoExtractor} = pluginConfig
-  const overrides = typeof pluginConfig.collections?.invoices === 'object' ? pluginConfig.collections?.invoices : {}
-  let fields: Field[] = [
+
+  // Get slugs for relationships - these need to be determined before building fields
+  const paymentsSlug = extractSlug(pluginConfig.collections?.payments, defaults.paymentsCollection)
+  const invoicesSlug = extractSlug(pluginConfig.collections?.invoices, defaults.invoicesCollection)
+
+  const fields: Field[] = [
     {
       name: 'number',
       type: 'text',
@@ -33,7 +39,7 @@ export function createInvoicesCollection(pluginConfig: BillingPluginConfig): Col
         position: 'sidebar' as const,
         description: 'Link to customer record (optional)',
       },
-      relationTo: extractSlug(customerRelationSlug),
+      relationTo: customerRelationSlug as any,
       required: false,
     }] : []),
     // Basic customer info fields (embedded)
@@ -277,7 +283,7 @@ export function createInvoicesCollection(pluginConfig: BillingPluginConfig): Col
         condition: (data) => data.status === 'paid',
         position: 'sidebar',
       },
-      relationTo: extractSlug(pluginConfig.collections?.payments || defaults.paymentsCollection),
+      relationTo: paymentsSlug,
     },
     {
       name: 'notes',
@@ -294,11 +300,9 @@ export function createInvoicesCollection(pluginConfig: BillingPluginConfig): Col
       },
     },
   ]
-  if (overrides?.fields) {
-    fields = overrides.fields({defaultFields: fields})
-  }
-  return {
-    slug: extractSlug(pluginConfig.collections?.invoices || defaults.invoicesCollection),
+
+  const baseConfig: CollectionConfig = {
+    slug: invoicesSlug,
     access: {
       create: ({ req: { user } }: AccessArgs) => !!user,
       delete: ({ req: { user } }: AccessArgs) => !!user,
@@ -313,10 +317,68 @@ export function createInvoicesCollection(pluginConfig: BillingPluginConfig): Col
     fields,
     hooks: {
       afterChange: [
-        ({ doc, operation, req }) => {
+        async ({ doc, operation, req, previousDoc }) => {
+          const logger = createContextLogger(req.payload, 'Invoices Collection')
+
           if (operation === 'create') {
-            const logger = createContextLogger(req.payload, 'Invoices Collection')
             logger.info(`Invoice created: ${doc.number}`)
+
+            // If invoice has a linked payment, update the payment to link back to this invoice
+            if (doc.payment) {
+              try {
+                const paymentId = typeof doc.payment === 'object' ? doc.payment.id : doc.payment
+
+                logger.info(`Linking payment ${paymentId} back to invoice ${doc.id}`)
+
+                // eslint-disable-next-line @typescript-eslint/no-explicit-any
+                await req.payload.update({
+                  collection: paymentsSlug as CollectionSlug,
+                  id: paymentId,
+                  data: {
+                    invoice: doc.id,
+                  } as any,
+                })
+
+                logger.info(`Payment ${paymentId} linked to invoice ${doc.id}`)
+              } catch (error) {
+                logger.error(`Failed to link payment to invoice: ${String(error)}`)
+                // Don't throw - invoice is already created
+              }
+            }
+          }
+
+          // If invoice status changes to paid, ensure linked payment is also marked as paid
+          const statusChanged = operation === 'update' && previousDoc && previousDoc.status !== doc.status
+          if (statusChanged && doc.status === 'paid' && doc.payment) {
+            try {
+              const paymentId = typeof doc.payment === 'object' ? doc.payment.id : doc.payment
+
+              // Fetch the payment to check its status
+              // eslint-disable-next-line @typescript-eslint/no-explicit-any
+              const payment = await req.payload.findByID({
+                collection: paymentsSlug as CollectionSlug,
+                id: paymentId,
+              }) as any
+
+              // Only update if payment is not already in a successful state
+              if (payment && !['paid', 'succeeded'].includes(payment.status)) {
+                logger.info(`Invoice ${doc.id} marked as paid, updating payment ${paymentId}`)
+
+                // eslint-disable-next-line @typescript-eslint/no-explicit-any
+                await req.payload.update({
+                  collection: paymentsSlug as CollectionSlug,
+                  id: paymentId,
+                  data: {
+                    status: 'succeeded',
+                  } as any,
+                })
+
+                logger.info(`Payment ${paymentId} marked as succeeded`)
+              }
+            } catch (error) {
+              logger.error(`Failed to update payment status: ${String(error)}`)
+              // Don't throw - invoice update is already complete
+            }
           }
         },
       ] satisfies CollectionAfterChangeHook<Invoice>[],
@@ -353,7 +415,7 @@ export function createInvoicesCollection(pluginConfig: BillingPluginConfig): Col
                 }
               } catch (error) {
                 const logger = createContextLogger(req.payload, 'Invoices Collection')
-                logger.error(`Failed to extract customer info: ${error}`)
+                logger.error(`Failed to extract customer info: ${String(error)}`)
                 throw new Error('Failed to extract customer information')
               }
             }
@@ -429,4 +491,12 @@ export function createInvoicesCollection(pluginConfig: BillingPluginConfig): Col
     },
     timestamps: true,
   }
+
+  // Apply collection extension function if provided
+  const collectionConfig = pluginConfig.collections?.invoices
+  if (typeof collectionConfig === 'object' && collectionConfig.extend) {
+    return collectionConfig.extend(baseConfig)
+  }
+
+  return baseConfig
 }
diff --git a/src/collections/payments.ts b/src/collections/payments.ts
index c6263e5..942c005 100644
--- a/src/collections/payments.ts
+++ b/src/collections/payments.ts
@@ -1,13 +1,18 @@
-import type { AccessArgs, CollectionBeforeChangeHook, CollectionConfig, Field } from 'payload'
+import type { AccessArgs, CollectionAfterChangeHook, CollectionBeforeChangeHook, CollectionConfig, Field } from 'payload'
 import type { BillingPluginConfig} from '../plugin/config';
 import { defaults } from '../plugin/config'
 import { extractSlug } from '../plugin/utils'
 import type { Payment } from '../plugin/types/payments'
 import { initProviderPayment } from './hooks'
+import { createContextLogger } from '../utils/logger'
 
 export function createPaymentsCollection(pluginConfig: BillingPluginConfig): CollectionConfig {
-  const overrides = typeof pluginConfig.collections?.payments === 'object' ? pluginConfig.collections?.payments : {}
-  let fields: Field[] = [
+  // Get slugs for relationships - these need to be determined before building fields
+  const invoicesSlug = extractSlug(pluginConfig.collections?.invoices, defaults.invoicesCollection)
+  const refundsSlug = extractSlug(pluginConfig.collections?.refunds, defaults.refundsCollection)
+  const paymentsSlug = extractSlug(pluginConfig.collections?.payments, defaults.paymentsCollection)
+
+  const fields: Field[] = [
     {
       name: 'provider',
       type: 'select',
@@ -79,7 +84,7 @@ export function createPaymentsCollection(pluginConfig: BillingPluginConfig): Col
       admin: {
         position: 'sidebar',
       },
-      relationTo: extractSlug(pluginConfig.collections?.invoices || defaults.invoicesCollection),
+      relationTo: invoicesSlug,
     },
     {
       name: 'metadata',
@@ -104,7 +109,7 @@ export function createPaymentsCollection(pluginConfig: BillingPluginConfig): Col
         readOnly: true,
       },
       hasMany: true,
-      relationTo: extractSlug(pluginConfig.collections?.refunds || defaults.refundsCollection),
+      relationTo: refundsSlug,
     },
     {
       name: 'version',
@@ -116,12 +121,10 @@ export function createPaymentsCollection(pluginConfig: BillingPluginConfig): Col
       index: true, // Index for optimistic locking performance
     },
   ]
-  if (overrides?.fields) {
-    fields = overrides?.fields({defaultFields: fields})
-  }
-  return {
-    slug: extractSlug(pluginConfig.collections?.payments || defaults.paymentsCollection),
-    access: overrides?.access || {
+
+  const baseConfig: CollectionConfig = {
+    slug: paymentsSlug,
+    access: {
       create: ({ req: { user } }: AccessArgs) => !!user,
       delete: ({ req: { user } }: AccessArgs) => !!user,
       read: ({ req: { user } }: AccessArgs) => !!user,
@@ -131,10 +134,43 @@ export function createPaymentsCollection(pluginConfig: BillingPluginConfig): Col
       defaultColumns: ['id', 'provider', 'status', 'amount', 'currency', 'createdAt'],
       group: 'Billing',
       useAsTitle: 'id',
-      ...overrides?.admin
     },
     fields,
     hooks: {
+      afterChange: [
+        async ({ doc, operation, req, previousDoc }) => {
+          const logger = createContextLogger(req.payload, 'Payments Collection')
+
+          // Only process when payment status changes to a successful state
+          const successStatuses = ['paid', 'succeeded']
+          const paymentSucceeded = successStatuses.includes(doc.status)
+          const statusChanged = operation === 'update' && previousDoc && previousDoc.status !== doc.status
+
+          if (paymentSucceeded && (operation === 'create' || statusChanged)) {
+            // If payment has a linked invoice, update the invoice status to paid
+            if (doc.invoice) {
+              try {
+                const invoiceId = typeof doc.invoice === 'object' ? doc.invoice.id : doc.invoice
+
+                logger.info(`Payment ${doc.id} succeeded, updating invoice ${invoiceId} to paid`)
+
+                await req.payload.update({
+                  collection: invoicesSlug,
+                  id: invoiceId,
+                  data: {
+                    status: 'paid',
+                  },
+                })
+
+                logger.info(`Invoice ${invoiceId} marked as paid`)
+              } catch (error) {
+                logger.error(`Failed to update invoice status: ${error}`)
+                // Don't throw - we don't want to fail the payment update
+              }
+            }
+          }
+        },
+      ] satisfies CollectionAfterChangeHook<Payment>[],
       beforeChange: [
         async ({ data, operation, req, originalDoc }) => {
           if (operation === 'create') {
@@ -167,4 +203,12 @@ export function createPaymentsCollection(pluginConfig: BillingPluginConfig): Col
     },
     timestamps: true,
   }
+
+  // Apply collection extension function if provided
+  const collectionConfig = pluginConfig.collections?.payments
+  if (typeof collectionConfig === 'object' && collectionConfig.extend) {
+    return collectionConfig.extend(baseConfig)
+  }
+
+  return baseConfig
 }
diff --git a/src/collections/refunds.ts b/src/collections/refunds.ts
index cfc6da4..daba5ec 100644
--- a/src/collections/refunds.ts
+++ b/src/collections/refunds.ts
@@ -6,9 +6,12 @@ import type { Payment } from '../plugin/types/index'
 import { createContextLogger } from '../utils/logger'
 
 export function createRefundsCollection(pluginConfig: BillingPluginConfig): CollectionConfig {
-  // TODO: finish collection overrides
-  return {
-    slug: extractSlug(pluginConfig.collections?.refunds || defaults.refundsCollection),
+  // Get slugs for relationships - these need to be determined before building fields
+  const paymentsSlug = extractSlug(pluginConfig.collections?.payments, defaults.paymentsCollection)
+  const refundsSlug = extractSlug(pluginConfig.collections?.refunds, defaults.refundsCollection)
+
+  const baseConfig: CollectionConfig = {
+    slug: refundsSlug,
     access: {
       create: ({ req: { user } }: AccessArgs) => !!user,
       delete: ({ req: { user } }: AccessArgs) => !!user,
@@ -37,7 +40,7 @@ export function createRefundsCollection(pluginConfig: BillingPluginConfig): Coll
         admin: {
           position: 'sidebar',
         },
-        relationTo: extractSlug(pluginConfig.collections?.payments || defaults.paymentsCollection),
+        relationTo: paymentsSlug,
         required: true,
       },
       {
@@ -120,13 +123,13 @@ export function createRefundsCollection(pluginConfig: BillingPluginConfig): Coll
             try {
               const payment = await req.payload.findByID({
                 id: typeof doc.payment === 'string' ? doc.payment : doc.payment.id,
-                collection: extractSlug(pluginConfig.collections?.payments || defaults.paymentsCollection),
+                collection: paymentsSlug,
               }) as Payment
 
               const refundIds = Array.isArray(payment.refunds) ? payment.refunds : []
               await req.payload.update({
                 id: typeof doc.payment === 'string' ? doc.payment : doc.payment.id,
-                collection: extractSlug(pluginConfig.collections?.payments || defaults.paymentsCollection),
+                collection: paymentsSlug,
                 data: {
                   refunds: [...refundIds, doc.id],
                 },
@@ -159,4 +162,12 @@ export function createRefundsCollection(pluginConfig: BillingPluginConfig): Coll
     },
     timestamps: true,
   }
+
+  // Apply collection extension function if provided
+  const collectionConfig = pluginConfig.collections?.refunds
+  if (typeof collectionConfig === 'object' && collectionConfig.extend) {
+    return collectionConfig.extend(baseConfig)
+  }
+
+  return baseConfig
 }
diff --git a/src/plugin/config.ts b/src/plugin/config.ts
index cfe468c..a475be8 100644
--- a/src/plugin/config.ts
+++ b/src/plugin/config.ts
@@ -41,6 +41,14 @@ export interface CustomerInfoExtractor {
   }
 }
 
+// Collection configuration type
+export type CollectionExtension =
+  | string
+  | {
+      slug: string
+      extend?: (config: CollectionConfig) => CollectionConfig
+    }
+
 // Plugin configuration
 export interface BillingPluginConfig {
   admin?: {
@@ -48,9 +56,9 @@ export interface BillingPluginConfig {
     dashboard?: boolean
   }
   collections?: {
-    invoices?: string | (Partial<CollectionConfig> & {fields?: FieldsOverride})
-    payments?: string | (Partial<CollectionConfig> & {fields?: FieldsOverride})
-    refunds?: string | (Partial<CollectionConfig> & {fields?: FieldsOverride})
+    invoices?: CollectionExtension
+    payments?: CollectionExtension
+    refunds?: CollectionExtension
   }
   customerInfoExtractor?: CustomerInfoExtractor // Callback to extract customer info from relationship
   customerRelationSlug?: string // Customer collection slug for relationship
diff --git a/src/plugin/utils.ts b/src/plugin/utils.ts
index 9d81725..274b886 100644
--- a/src/plugin/utils.ts
+++ b/src/plugin/utils.ts
@@ -1,10 +1,23 @@
 import type { CollectionConfig, CollectionSlug, Field } from 'payload'
 import type { Id } from './types/index'
+import type { CollectionExtension } from './config'
 
 export type FieldsOverride = (args: { defaultFields: Field[] }) => Field[]
 
-export const extractSlug =
-  (arg: string | Partial<CollectionConfig>) => (typeof arg === 'string' ? arg : arg.slug!) as CollectionSlug
+/**
+ * Extract the slug from a collection configuration
+ * Returns the slug from the configuration or the default slug if not provided
+ */
+export const extractSlug = (arg: CollectionExtension | undefined, defaultSlug: string): CollectionSlug => {
+  if (!arg) {
+    return defaultSlug as CollectionSlug
+  }
+  if (typeof arg === 'string') {
+    return arg as CollectionSlug
+  }
+  // arg is an object with slug property
+  return arg.slug as CollectionSlug
+}
 
 /**
  * Safely cast ID types for PayloadCMS operations
diff --git a/src/providers/utils.ts b/src/providers/utils.ts
index 9803177..a6e436a 100644
--- a/src/providers/utils.ts
+++ b/src/providers/utils.ts
@@ -35,7 +35,7 @@ export async function findPaymentByProviderId(
   providerId: string,
   pluginConfig: BillingPluginConfig
 ): Promise<Payment | null> {
-  const paymentsCollection = extractSlug(pluginConfig.collections?.payments || defaults.paymentsCollection)
+  const paymentsCollection = extractSlug(pluginConfig.collections?.payments, defaults.paymentsCollection)
 
   const payments = await payload.find({
     collection: paymentsCollection,
@@ -59,7 +59,7 @@ export async function updatePaymentStatus(
   providerData: ProviderData<any>,
   pluginConfig: BillingPluginConfig
 ): Promise<boolean> {
-  const paymentsCollection = extractSlug(pluginConfig.collections?.payments || defaults.paymentsCollection)
+  const paymentsCollection = extractSlug(pluginConfig.collections?.payments, defaults.paymentsCollection)
 
   try {
     // First, fetch the current payment to get the current version
@@ -141,7 +141,7 @@ export async function updateInvoiceOnPaymentSuccess(
 ): Promise<void> {
   if (!payment.invoice) {return}
 
-  const invoicesCollection = extractSlug(pluginConfig.collections?.invoices || defaults.invoicesCollection)
+  const invoicesCollection = extractSlug(pluginConfig.collections?.invoices, defaults.invoicesCollection)
   const invoiceId = typeof payment.invoice === 'object'
     ? payment.invoice.id
     : payment.invoice