It’s a popular question on the forums: How can I do e-commerce with ExpressionEngine?
The only built-in way to do e-commerce is using the Simple Commerce Module (SCM), a first-party module from EllisLab. This method is, of course, limited to using PayPal as a payment gateway and some clients are hesitant to do that because they (incorrectly) perceive PayPal to be an amateur solution and not secure. Also, SCM has limited functionality; it’s called Simple Commerce Module for a reason.
One of the features that is not in SCM is the ability to sell subscriptions. This is a frequent request on the forums and as of right now the only tool that is integrated with EE to do this is FoxEE by HCC Development.
FoxEE is an ExpressionEngine module that bridges the gap between EE and the Foxycart hosted e-commerce tool. With FoxEE you can use Foxycart to sell subscriptions, downloads or physical goods. It can also apply discounts to member groups. You can get a better idea of how FoxEE works by reading through the documentation.
Those are the only two integrated e-commerce solutions for ExpressionEngine that I know of. What am I missing? Have you used any others?