Normally you sign a contract with a PSP (Payment Service Provider)
which will give you access to its gateway. With this gateway, you can access
all the payment services that you've booked for integration
into your web services.

(See also http://en.wikipedia.org/wiki/Payment_service_provider )

Most - if not all - do charge a monthly fee for the basic service
and gateway access, and an additional fee for every transaction
you handle over their gateway.