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

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?