I need to get a e-commerce site up and running, but I really don't want to program it from the bottom up if I don't need to. I want to program it using ASP.NET MVC.

I'm looking for a good open source alternative (or one for purchase if it's modular enough) that I can use as a base and enhance with the needed functions? 

It has to have all "normal" e-commerce functions, and also the possibility to integrate with a credit card API of my choice.

If anyone have any recommendations for me here, I would appreciate it :)