...here's something to think about: a shopping cart does not process credit cards.
In much the same way a real-life shopping cart doesn't accept payment, neither does an online one.
There are actually two separate services: the shopping cart software and the payment processor.
Just to confuse things, some payment processors have their own built-in shopping cart functionality. PayPal does this. That's great, but it's not always the best solution for people's needs.
If you're interested in selling things online, you'll need to think about both of these items.