osCommerce can offer a ton of functionality, there’s a good community of contributors and support, but it’s not easy at all to customize the look of your site. Also, if you’re into standards based design, osCommerce is all table based layout and very time consuming if you want to try and switch it to a complete CSS based approach.
I can’t think of a truly free option that is easy to change the look and feel of although there’s so many I may have missed one.
The closest I’ve had experience with is Avactis and I’ve never used it for a full project. Just installed it and played around a bit. It’s tag based so you can theoretically work it into any design you’d like. I really liked the flexibility of it, it still generates tables in some areas but you could handle the bulk of the design yourself. I seem to remember they offered a free copy to web designers (the first one is free, then you pay sort of thing) but I didn’t see that on the site right now. They do have a slightly limited version you can download and try, if you like it, it’s $200 for the real deal.
ZenCart is another free option that’s basically an offshoot of osCommerce. Some people like it quite a bit better so it may be worth a look as well. I’ve never used it.
A couple to keep your eyes on for the future would be freeCSScart and MagentoCommerce both of which look very promising, but aren’t ready for prime time yet. Magento has a beta out and freeCSScart hasn’t even gotten there. But if you’re going to be doing a lot of ecommerce sites it may be worth a look so you can dream of how things could be.
Good luck with the client and the site.