refactor: Replace conditional fields with customer info extractor callback

- Add CustomerInfoExtractor callback type for flexible customer data extraction
- Implement automatic customer info sync via beforeChange hook
- Make customer info fields read-only when using extractor
- Add defaultCustomerInfoExtractor for built-in customer collection
- Update validation to require customer selection when using extractor
- Keep customer info in sync when relationship changes

Breaking change: Plugin users must now provide customerInfoExtractor callback
to enable customer relationship syncing.

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

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

dev/payload.config.ts

@@ -2,7 +2,7 @@ import { sqliteAdapter } from '@payloadcms/db-sqlite'
 import { lexicalEditor } from '@payloadcms/richtext-lexical'
 import path from 'path'
 import { buildConfig } from 'payload'
-import { billingPlugin } from '../dist/index.js'
+import { billingPlugin, defaultCustomerInfoExtractor } from '../dist/index.js'
 import sharp from 'sharp'
 import { fileURLToPath } from 'url'
 
@@ -61,7 +61,23 @@ const buildConfigWithSQLite = () => {
           refunds: 'refunds',
           // customerRelation: false, // Set to false to disable customer relationship in invoices
           // customerRelation: 'clients', // Or set to a custom collection slug
-        }
+        },
+        // Use the default extractor for the built-in customer collection
+        customerInfoExtractor: defaultCustomerInfoExtractor,
+        // Or provide a custom extractor for your own customer collection structure:
+        // customerInfoExtractor: (customer) => ({
+        //   name: customer.fullName,
+        //   email: customer.contactEmail,
+        //   phone: customer.phoneNumber,
+        //   company: customer.companyName,
+        //   taxId: customer.vatNumber,
+        //   billingAddress: {
+        //     line1: customer.billing.street,
+        //     city: customer.billing.city,
+        //     postalCode: customer.billing.zip,
+        //     country: customer.billing.countryCode,
+        //   }
+        // })
       }),
     ],
     secret: process.env.PAYLOAD_SECRET || 'test-secret_key',