Wow... what a bunch of tables. You have scrapped most of the Zen Cart layout and wrapped it in your custom table setup, when it could have looked exactly the same using the stock divs and a few new wrapper divs.

That aside, it appears you are missing a </div> for #headerWrapper, as it goes down to the very bottom of the ZC-generated part of the page. Fix that and see if it makes a difference.

Most of the tables are your own creation, and we can't see your code directly to troubleshoot.
Oh, and when you say "Powered by SimpleSage", did you know that is a claim to have written the engine that runs your shop? You can remove the "Powered by Zen Cart", but you can't substitute your own. The copyright is definitely yours in this case.