Acts_as_authenticated and paypal

I’m working on a subscription form that has to be tied to paypal using
the paypal ipn plugin at http://elctech.com/products_ruby_paypal.shtml.

I’ve got my subscription (signup) form setup to include the creditcard
information as well as the billing address etc. It’s a modified version
of the signup form from the acts_as_auth plugin.

I want the model to not save if the paypal direct payment does not
return 'success". Do I create a before_save filter for that or what is
the best method to use?