Merchant Webhook Endpoint Ratepay Payment API Ratepay HPP Frontend Ratepay HPP Session API Ratepay Auth API Merchant Backend Merchant Frontend Customer Merchant Webhook Endpoint Ratepay Payment API Ratepay HPP Frontend Ratepay HPP Session API Ratepay Auth API Merchant Backend Merchant Frontend Customer order_data + url_data (success/cancel/failure/webhook) session_id + landing_page_url COMPLETED / FAILED / CANCELLED After shipment Invoice + customer communication Open checkout page 1 Render checkout, show Ratepay option 2 Select Ratepay 3 Enter consent + click Buy 4 Submit order + buyer/basket info 5 Request OAuth token 6 Return Bearer token 7 POST /pps/api/v1/session 8 201 Created 9 Send landing_page_url 10 Redirect to Ratepay HPP 11 Open landing_page_url 12 Render payment form 13 Enter payment data & confirm 14 Validate & process payment 15 Success / Cancel / Failure page 16 Redirect back via success/cancel/failure URL 17 POST session_status event 18 Update order status 19 POST /captures 20 Capture confirmed 21 POST shipping info 22 POST /cancellation optional 23 POST /refunds optional 24