Just had the same issue and had to pay 99$ to wp-ecommerce to get this simple answer so, hope it helps you or someone else. First, I needed to upgrade both wp-ecommerce and gold cart to latest version. This should have fixed the issue of the page just refreshing upon purchase selection(which was related to the new authorize.net security roll out) but lastly needed to update my version of php on the server in order for the changes to work correctly on the updated plugins. Hope this helps.
