Ultimately I decided it was not worth it to rescue the old ruby on rails site, and I decided to use an off-the-shelf store (Magento). There are pros and cons to it... inventory management is really smooth, and I had to do relatively little work to get it working, but it means tweaking everything, a lot, and I still don't get quite exactly what I want. My biggest gripe is with the checkout process; it has like 3 too many steps, and some bad defaults. I'd like to try to streamline it if I get the chance, but who am I kidding? The only way this store is ever gonna see more work is if I actually sell all the shirts I already have in stock and decide to do another run.
But anyway now you can vote for which food-on-shirt related flash game I should make! You''ll see the poll if you go to one of the shirt pages, e.g. here and scroll down and check out the sidebar.