Implement Apple Pay merchant validation and decrypt the resulting payment token server-side

Steps

1. On receiving the onvalidatemerchant event (web) or the equivalent native delegate callback, send the provided validationURL to your server.
2. From your server, POST to that validation URL over mutual TLS using your Apple merchant identity certificate to request a merchant session; use an allow-list of Apple's validation domains rather than hardcoding a single endpoint, since Apple's own guidance is moving from the older 'Start Session' terminology to 'Payment Session.'
3. Return the merchant session to the client so it can call completeMerchantValidation and unblock the payment sheet.
4. After the user authorizes payment, receive the resulting payment token (ApplePayPaymentToken or PKPaymentToken) containing paymentData with data, header (ephemeralPublicKey, publicKeyHash, transactionId), signature, and version.
5. Either decrypt the token yourself server-side (ECDH with the ephemeral public key, then AES-256, per Apple's Payment Token Format Reference, using your payment processing certificate's private key) or forward the opaque token to your payment processor, which is the more commonly used integration path.