The only real way to do that is to store the session variable in the URL. And that comes with its own host of issues.
People are going to be pissed when they load up their cart and then follow some bookmark they created for a product on your site to add it to the cart, only to find the cart empty.
People are going to be pissed when they load up their cart and then follow some bookmark they created for a product on your site to add it to the cart, only to find the cart empty.