This project is pretty extensive. Hosted at Sourceforge. Features a real admin panel, template engine and basically this means a full featured shopping and order management solution.
Lots of things to get going, I had some trouble with paths, but for instance. Still - sourcecode is easily read and it therefore would allow you to do some pretty nice customizations if needed. I never tested it out all the way, cause for my particular solution it was too heavy. But this one sure is recommended to try out.
Important: after you downloaded, the userid for admin panel (accessible through /admin(index.asp) is administrator with password admin.
A shop