one thing i notice strait off is that your site will redirect the addresses:
but the address:
does not redirect.
this is most likely the cause of the problem.
the cart is stored in a session, so if a customer starts browsing your site using the address:
and then gets direxted to:
it will be a new browsing session and cause the cart to be emtpy.
you can use an htaccess rule with mod rewrite to prevent this from happening: