This issue has been fixed and cart v3.88 is availible on the downloads page. it turns out that dnn in their infinite wisdom changed the navigateurl function in version 5.5.x and either broke or discontinued one of the overloads the cart used to pass the productid in the querystring. See the following url for more description of the issue and thanks to Tony for helping to fix it.
http://www.dotnetnuke.com/Community/F...