Passing an additional variable from cart.php to checkout.php page

I'm using the standard process from the checkout wizzard to generate orders for a franchise website (ecart 5). Currently when it rolls from the cart.php page to checkout.php page, the billing and shipping info is pulled from the user table. They wish to change the system so that one user is can order product for multiple locations which means I need to pull shipping and billing from a locations table, not the users table.

I have a relational table established identifying what users have control over what locations. I have a recordset pulled on the cart.php page, and it populates a drop menu within the form on the cart.php page.

Can anyone offer some direction how to pass that form input variable from the cart.php page to the checkout.php where I could then theoretically query a recordset with that variable to populate the billing and shipping information properly? The checkout process doesn't pass these variables in a URL string, which is what I'm familiar with.

I've attached the cart.php in case you need to reference it.