Create checkout on your server
Your backend requests a checkout session using authenticated merchant context and a valid product or price reference already configured for your merchant scope.
Keep pricing authoritative
The browser can choose an eligible product, but financial amounts, currency rules, and merchant ownership should be resolved and frozen by trusted server-side logic.
After payment
Use the canonical order and connected payment state as authority. Signed events can notify your merchant systems when material order, payment, refund, shipment, or subscription state changes.
Customer return URLs
A successful browser return can improve the user experience, but it should not be treated as the only proof that a payment completed.