Say I have two email addresses X and Y. I normally use X and keep Y for more private uses, such as Paypal. Somebody mistakenly sent me a payment to the X email address. I could tell them to cancel the payment, but I'm afraid that if they did so Paypal would still keep their cut.
If they do, I'd very much rather avoid that and sign up with the X email address and just pay something the amount of money received precisely, something that's not too hard nowadays thanks to the "Pay what you want" offers.
So: does Paypal keep the cut on a cancelled payment? The help center does not answer this question in the relevant support article.