actually the "authentication for paypal express checkout" server behaviour is on the same page as the registration page. But once it is fired (I've set the trigger to activate on registration button click) the paypal page will appear waiting for me to login and pay.
I would like the user to be redirected to a confirm page where he can check the entered data and then click the PAY button.
I've not yet understood how this should work with cart.
Sorry for my incompetence.