Marketpress-Items replace each other in cart rather than adding

I'm using a customized version of Gridmarket/Framemarket with MarketPress here: http://packers.hotdinosaur.com and I'm running into an issue that I'm stuck on. If I add an item to an empty cart, it adds just fine. However, adding an item to a cart with anything at all in it will cause the carts contents to be replaced by the incoming item. This is reflected in real time with the AJAX widget. Please let me know what I can do to add multiple items to the cart. Thanks!