feat: add checkoutUrl field to payment collection

- Add checkoutUrl field to Payment type and collection
- Mollie provider now sets checkoutUrl from _links.checkout.href
- Test provider sets checkoutUrl to interactive payment UI
- Stripe provider doesn't use checkoutUrl (uses client_secret instead)
- Update README with checkoutUrl examples and clarifications
- Make it easier to redirect users to payment pages

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/collections/payments.ts

@@ -78,6 +78,14 @@ export function createPaymentsCollection(pluginConfig: BillingPluginConfig): Col
         description: 'Payment description',
       },
     },
+    {
+      name: 'checkoutUrl',
+      type: 'text',
+      admin: {
+        description: 'Checkout URL where user can complete payment (if applicable)',
+        readOnly: true,
+      },
+    },
     {
       name: 'invoice',
       type: 'relationship',

src/plugin/types/payments.ts

@@ -22,6 +22,10 @@ export interface Payment {
    * Payment description
    */
   description?: string | null;
+  /**
+   * Checkout URL where user can complete payment (if applicable)
+   */
+  checkoutUrl?: string | null;
   invoice?: (Id | null) | Invoice;
   /**
    * Additional metadata for the payment

src/providers/mollie.ts

@@ -155,6 +155,7 @@ export const mollieProvider = (mollieConfig: MollieProviderConfig & {
       });
       payment.providerId = molliePayment.id
       payment.providerData = molliePayment.toPlainObject()
+      payment.checkoutUrl = molliePayment._links?.checkout?.href || null
       return payment
     },
   } satisfies PaymentProvider

src/providers/test.ts

@@ -492,6 +492,7 @@ export const testProvider = (testConfig: TestProviderConfig) => {
 
       // Set provider ID and data
       payment.providerId = testPaymentId
+      const paymentUrl = `${baseUrl}/api/payload-billing/test/payment/${testPaymentId}`
       const providerData: ProviderData = {
         raw: {
           id: testPaymentId,
@@ -500,7 +501,7 @@ export const testProvider = (testConfig: TestProviderConfig) => {
           description: payment.description,
           status: 'pending',
           testMode: true,
-          paymentUrl: `${baseUrl}/api/payload-billing/test/payment/${testPaymentId}`,
+          paymentUrl,
           scenarios: scenarios.map(s => ({ id: s.id, name: s.name, description: s.description })),
           methods: Object.entries(PAYMENT_METHODS).map(([key, value]) => ({
             id: key,
@@ -512,6 +513,7 @@ export const testProvider = (testConfig: TestProviderConfig) => {
         provider: 'test'
       }
       payment.providerData = providerData
+      payment.checkoutUrl = paymentUrl
 
       return payment
     },