Have you thought about using Paypal. They provide a number of services including account validation, fraud protection, and so on. Also there are test websites (i.e., eliteweaver.com) where you can test your shopping cart.
There are quite a few variables that can be passed back and forth between the payment processing server and the web server (i.e., your server).
It's easy to use.
If you're interested, I've placed a number of entries in this forum: search for Paypal, eliteweaver, etc.