We would like to be able to accept credit-card payments for some online products we are going to make available. We already have a user-management and registration system. We really don't need a storefront application, but rather just a way to process a credit-card payment and get some kind of real-time notification from the payment processing system that the payment went through. The usual stuff. What's the most straightforward way to get something like this up and running? Thanks very much.