How do donations work, exactly?

[DavidJCobb]

I know that we input PayPal details into the Nexus, but beyond that, the whole donation system's a black box to me.

 

I'm not familiar with PayPal, but I know people who are. They tell me that there are multiple types of transactions, and that PayPal usually takes a cut of the cash as a processing fee. There are some types of transactions that a sender can revoke, and in these cases the recipient gets nothing but still has to pay the fee. Are Nexus donations vulnerable to this?