Is there a module that provides payment processing but not a shopping cart?
Have you tried some of those ?


lm_paypal: 


  This module provides an interface to PayPal using Instant Payment
  Notifications (IPN). It currently supports paid memberships
  (subscriptions), donations (tip jar) and paid adverts (classified
  ads).

Payment API: this one need a specific module for the payment platform


  This module can be used by other modules to collect and track payments. Through this API, any module can provide a payment form and payments can be handled and tracked centrally.

How to build a paid membership site with Drupal 7 (paid module)