During the checkout process, all the active payment method rules will be evaluated and given a chance to enable their respective payment methods for use on the checkout form.You can actually use more than one rule to enable any given payment method using a different set of API credentials or transaction settings based on conditions of the order being paid for.
The file commerce_payment.credit_includes helper functions for building and validating credit card forms and data.Payment method modules integrating with credit card processing services should use these functions and can use the Commerce Authorize. Whenever a payment is attempted, a payment transaction should be created that references the order for which payment was attempted.This includes transaction attempts that failed, that require further action, or were processed successfully as reflected in the transaction's status: Failure, Pending, Success.There are two broad categories of payment methods enabled by the payment system out of the box: on-site payment and off-site payment.On-site payment is either processed via a third party web service using data collected through a checkout or administration form or simply entails presenting payment information to the customer for them to remit payment offline upon checkout completion.
Off-site payment is enabled through a redirect from the Payment checkout page to the payment service, with customers ideally being returned back to the Payment page upon success or failure so they can be moved forward or backward in the checkout process as the case may require.
Because off-site payment often requires direct customer input (such as a username and password), it is not normally possible to enact this type of payment through the administration form.
Payment gateways tend to offer multiple types of payment services, often providing on-site and off-site options.
Each service should be represented by a different payment method in Drupal Commerce.
Using Pay Pal as an example, this means the Commerce Pay Pal project will include modules defining their services a separate payment methods, including Pay Pal WPS (off-site service), WPP (on-site service), and EC (a mixture between off-site and on-site).
When a payment method module is enabled, a default payment method rule will be defined that you can use to configure settings for your payment methods.