It might be overkill for what you need, but the open source Freeway (http://www.openfreway.org http://www.openfreway.org) ecommerce platform does that natively.

It might be overkill for what you need, but the open source Freeway (http://www.openfreway.org http://www.openfreway.org) ecommerce platform does that natively. There are also a number of bridges for Joomla! to bring specific Freeway functions over without wrapping the whole thing into Joomla!.