[Pandiyan5273]

Issue

• The Webshop order page sends the Pay action as GET, but ERPNext's payment-request endpoint requires POST. Customers cannot start payment from that button, and partially paid invoices display the original total instead of the remaining balance.
• Refs Guest checkout fails on Shopping Cart payment request with PermissionError after upgrade (v16) erpnext#52844. This covers authenticated customer checkout; standard Webshop checkout continues to require login.

Steps to replicate

• Install Payments and Webshop, configure a payment gateway account, and enable checkout.
• Link a website user with the Customer role to a customer and submit an unpaid Sales Order for that customer.
• Sign in as that user, open the order page, and click Pay.
• Also open a submitted Sales Invoice for 1,000 with 400 already paid.

Actual behavior

• Pay sends GET to the POST-only endpoint and is rejected.
• The partially paid invoice displays Pay 1,000 even though only 600 remains payable.

Expected behavior

• Pay submits a POST form with the session CSRF token and redirects to the configured gateway.
• The button displays the payable balance and is hidden for draft, cancelled, fully paid, or unauthorized documents.

Backport needed

• Version 16: Yes. The same GET link exists at Webshop 5ba16567c0; the prepared version-16 companion also requires ERPNext's portal-customer permission fix. Both changes passed the v16 server flows described in their PRs.
• Version 15: Yes, by code inspection at Webshop a355cdad44 and ERPNext d17086df9f. The GET link remains and ERPNext lacks the customer permission path; coordinated adaptations and v15 runtime validation are still required.

Companion PRs

• Version-16 Webshop counterpart: fix(portal): submit checkout payments with CSRF protection (v16) #415.
• Required ERPNext v16 permission fix for that counterpart: fix(accounts): allow customers to pay their portal documents (v16) erpnext#64260. Current ERPNext develop already has the server authorization path.

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