Interkassa with a predefined payment method
 

Make it possible to specify a payment method (the ik_paysystem_alias parameter) in the billing interface. This will enable to define a payment system (and the redirect to that system) without displaying a payment method form to user.

http://www.interkassa.com/lib/paysys...php?format=xml
Documentation: http://www.interkassa.com/docs/interkassa.tech.doc