Sorry, been quite busy...
I noticed you are using an inline frame "<iframe>" to display the dynamic contents of your page. First I would like to suggest you use <div> instead of iFrame because of compability issues with iFrame.
I reckon the cart loads incorrectly because of the iFrame tag. There must be a refresh issue with nested frames (in combination with a js call).
Since I never use iFrame I cannot give you any further information. please point your browser to http://www.w3c.org
and search for iframe refresh issues with js calls.
I recommend that you test using the <div> tag as well, perhaps that solves the problem immediatly.
Hope this helps,