Looking for ecommerce system with just backside management, suggestions?

Back in 2006 I developed a tiny/custom e-commerce store for my wife using the Yahoo Merchant services. While this system works well as the most basic e-commerce system, it does lack some of the simplest features such as a quantity field and the allowance of .htaccess files. Frankly, I think the system is exactly the same as it was 6 years ago. Now my wife wants to expand her site and wants to include these aforementioned features to make the site a little more friendly to both users and search engines.

I have reviewed OpenCart and MagnetoGo, but these systems make you stick their front-end systems/templating. I don’t want to have learn a new template system if I don’t have to. The fact that I can just drop tags into pages which then interact with the Yahoo back-end is very attractive. I don’t want to manage the cart, store credit cards, or process orders.

My question is, are there any other systems out there like Yahoo’s, which allow you to take FULL control of the front-end (design & layouts) while at the same time the service manages the back-end (cart, processing, email confirmations, etc)?

Thanks in advance.