add to cart module

I use this module (add to cart) so that my registration page is at the same time a payment page for the membership that I sell. So once visitors check the button ‘aanmelden’ they go to a registration page (meestergijs.nl/checkout) and the product (my membership) is added. But after registration and payment and once they are logged in, it still shows that product in their cart. It’s causing a lot of questions. Some think they have to pay twice or something else went wrong.. How can I still use the module but not have the cart filled after registration/payment has been done?

Thanks,

Gijs (Holland)

The page I need help with: [log in to see the link]