Pass custom variable to PayPal Express Cart

I am building a site in Wordpress that uses [login to view URL]'s wp e-commerce plugin. Long story short - I need to append the design a customer chooses to the item name in the shopping cart. It cannot be a custom variant in the plugin because it will be part of the stock inventory control and I don't want or need to track design inventory, just the size and color of the shirt. I have already added the drop-down functionality to the product page, but I cannot seem to get the chosen value to pass to the PayPal Express cart. I want it viewable in both the sidebar cart and in the actual shopping cart page so the customer knows they chose the appropriate design and so I will receive that from PayPal as well. I have no doubt that this will be a very simple project for those familiar with PayPal and Wordpress