Unfortunately, I couldn't find any documentation for these services. It is possible that the APIs are published only for the merchants who are authorized to use them, payment gateways like CCAvenue and TPSL being some of them.
Banks/credit card companies have their payment APIs[0] open to parties they deal with (usually) payment gateways(now don't ask me for public methods and web service URIs) ->> payment gateway firms[1] deal with the banks and have access to those APIs ->> merchants/sellers[1] deal with payment gateways ->> customer checks out on seller/merchant's website ->> is redirected to (usually) payment gateway's portal where basic card/account credentials[2] are entered ->> is redirected bank/card's secure authentication page[3] ->> user enters login/password ->> is redirected back to seller/merchant's page with success/fail message.
[0] You might have to get in touch with some bank and/or payment gateway.
[1] When the seller is big, sometimes, they are themselves the payment gateways. Some PG examples are Citrus, Billdesk, SBI,CCAvenue(the shittiest) etc.
[2] Here in India the basic credentials include card number, exp date, name, CVV. In the USA these same basic details are more than enough for final purchase - sometimes even minus the cvv.
[3] Usually Mastercard's NetSafe or Verified By Visa etc.
