You mau have heard about Ecwid shopping cart extension for Joomla - you can find more information here.There are few advantages to Ecwid comparing to other similar products.It has AJAX supports drag-and-drop. Upgrades are seamless and it requires little to no technical support. Of course it totally free of charge. Ecwid works nice with a fresh Joomla installation. All you need to remember is to disable WYSIWYG/HTML editor before inserting the Ecwid code. This shopping cart has product catalog, Google Checkout, supports paypal and has many more graet features. Hope that you’ll find using this extension useful for you.