Salesforce Billing Payment Gateway Interfaces | Salesforce Billing Developer Guide

Creating a PaymentGateway Interface
The PaymentGateway interface contains several basic transaction methods that the PaymentGatewayAPI class implements to handle gateway requests. The methods allow PaymentGatewayAPI to create a credit card token, process a payment charge, void a payment, authorize a payment, and refund a payment.

Creating a PaymentGateways Interface
The PaymentGateways interface uses the processPayments method, which evaluates the payment transaction’s type to perform a payment action.

Creating a PaymentGatewayStatus Interface
Salesforce Billing implements the PaymentGatewayStatus interface to populate the TransactionResult’s GatewayStatus enum with the value of the return code that the external gateway set in transactionResult.