Mutation.storefrontBeginVnpayCheckout
Pre-create order in pending_payment status + build a signed VNPay payment URL. return_url is where VNPay redirects the browser back (e.g. https://shop/checkout/vnpay-return).
ReturnsVnpayCheckoutSession!
Signature
Pre-create order in pending_payment status + build a signed VNPay payment URL. return_url is where VNPay redirects the browser back (e.g. https://shop/checkout/vnpay-return).
session_idString!quote_idID!return_urlString!