Surely, there must be thousands of sites out there already written that want to simply add a cart? Is there really no way other than writing your own?
Never said that there were not other solutions....Just that Zen Cart is not by default modular.
One could "Custom Code" it to do what you want but as I said that would take many times more effort than making it look like your site. This making Zen Cart look like your site is not all that difficult.
In addition, as the pages are automatically assembled from many pieces of code you do not have 1,000's of pages to continually edit to make them current and if you have ever attempted to maintain a html site just a small change can mean editing many pages to keep it all functioning correctly and looking seamless.
Look at this complete site for instance. If you click Home at the top this takes you to Zen Cart , follow the the FAQ link and that is knowledge base or tree??, go to the wiki and I think that it is docuwiki, and right here is vBulletin.
Four apps and all essentially have the same look, but the pieces are not part of a buying experience, all that is handled in the cart side.