Create an Afterpay/Clearpay direct checkout session and capture payment using the Afterpay direct API (outside of Stripe)

Steps

1. Authenticate with Afterpay's API using HTTP Basic Auth with Merchant ID and Secret Key from the Afterpay merchant portal; endpoint is api.afterpay.com (global) or region-specific subdomains
2. POST /v2/checkouts with consumer (email, givenNames, surname), billing and shipping details, and items array including name, quantity, price.amount and price.currency; also include amount (total) and merchant.redirectConfirmUrl and merchant.redirectCancelUrl
3. The response contains a token and a redirectCheckoutUrl; redirect the customer to the redirectCheckoutUrl to complete the Afterpay installment agreement
4. On redirect return to redirectConfirmUrl with orderToken in query params, call POST /v2/payments/capture with token to finalize payment; the response includes payment id and status=APPROVED
5. Store the Afterpay payment id for refund and order management; initiate refunds via POST /v2/payments/<orderId>/refund with amount