Integration with Paypal shopping carts

I've finally got an attractive Paypal shopping cart (Simple eCommerce) but when I try to click the buy now button on a non-membership page it takes me to the protected content page (Membership).

Is there a way to allow the shopping cart page of the other plugin to appear and to finalize the purchase?

Here is the page with the purchase buttons:

Clicking on a button takes you here:

Obviously, I don't want this situation to last long... ;-(