Google Analytics goal for subscriptions

I would like to track and funnel the process of users and paid subscriptions.

I know this would be the first step: http://domain/payments.php

but what would I do after to verify they donated and to track conversion value.

These goal values are not built in like they are for registrations. The issue here is that they leave your site (to paypal, etc), so you can't track anything in the middle on the external domain.

I'm not sure if GA will allow for other urls in the middle to not count or not. Perhaps someone else has more advice on it than I.