Frédéric, from what I read it sounds like you want people to subscribe to access the site or certain areas of the site including the cart. If that's all you mean then you can probably just use DNN's built in payment system. Just create a role and give it a fee. People can select that role from the Manage Services page in their profile and they will be directed to the payment processor for payment. When they return they will have access to whatever you have granted that role.

The only issue is that you have to tell people to do it, because it doesnt' happen as part of the registration procedure. That is not really an issue if you have free and subscription sections on your site and people can make the choice to subscribe at some point after registering.

You can also use Ventrian's Subscription Tools to control content for subscribers. This handles payments and also conveniently handles notifications.

I'm not 100% sure what you're doing, but just wanted to make sure you don't do a lot more work than you need to.

Rob