Ask for user details at the end of the process
If I don't right away want to fill out my donor information and ignore it, there's currently no way to "go back" and leave it.
Regardless of this, I propose we move it to the end of the process, maybe even after the whole payment process to the "thank you" page. There, we could probably "auto-learn" and suggest the information we get from Paypal, and it feels more natural to me.
The downside is that maybe people don't bother leaving it when they already "finished" the donation, then later remember they do want some receipt, but at least the data collection should be moved to either right before payment, or right after. I argue that it can be done after payment. We need a separate form for people who "forgot" anyway, since people might have used a static donation code they got somewhere else.