I owe my success to having listened respectfully to the very best advice, and then going away and doing the exact opposite.
G K Chesterton

Zen Cart is one of the most popular open-source shopping carts, and with good reason. Although derived from the age-old OSCommerce application, Zen Cart’s code is much improved, notably including a system of file overrides that make upgrading relatively straightforward. (OSCommerce, by contrast, is extremely difficult to upgrade after any significant customisation has been done).

The appearance of the site can be customised with CSS changes only (simple but a bit limited) or by overriding the layout of any pages. Additional functionality is available through contributed plug-in modules (eg. for most popular payment processors and courier companies). Completely new modules can also be written as needed.

More information is available from the Zen Cart web site.