spacemanxpress

in my experience, os commerce is great... but it depends on what version you get and what sort of contributions you need. The one i've been using is one that was designed to work from the uk, so there are little things that aren't relevent to us customers and some that are just nods to the uk english speaking crowd.
also, if you need things like tax zones for NYC boroughs, or county taxes for the state of CA, it can get a bit tricky to navigate and you have to "jimmy rig" the back end.

OS commerce is good though, and if you can get the basic cart and template together, you can probably post an add on here, or craigslist to get somone to customize it further for you.
-Clint