It would be easier to start from scratch in my opinion. You can re-use the images and stuff, but the rest of it is a wreck. Did not validate very well at all. Over 70 errors, a web development company should not have these kind of mistakes. For me to have that many errors I would have to almost go out of my way. If you paid for it I would definitely ask for a refund, or go cuss them out like you were thinking.
If you can find some way to add the e-commerce stuff to your site, I have a 2 column tabless website layout that I made for a tutorial in the past. I can throw it your way to help you save some time, you would just need to resize some things.
Also you know there are free e-commerce cms that are very good. Such as magento, you can find a good amount of free templates for this as well, and just modify the header or something.