Gavo,1. Does you PayPal account have your Google Analytics tracking code on it? That's the first step, which may not be possible to implement.
2. If you get GA on the PayPal account, then you will have to pass cookies using the Auto Linker in Universal GA and exclude referral traffic from PayPal in the exclusions list like Peter suggested.
https://support.google.com/analytics/answer/1034342?hl=en
3. Even if you get this setup, I'm not sure it will work the way you want because you are going from your domain, to PayPal, and back to your domain. That 3 step process seems like it would break every time.
