Ok, this is academic as the parcel turned up at the last minute but:
Buyer paid by Paypal. Item doesn't arrive, so I agree full refund. I go to the payment and the 'Refund' button, enter the amount, and get an error message saying "You cannot make this payment at this time"
My Paypal balance might have been less than that, but I could still buy stuff as it just came off the linked credit card. So why couldn't I do a refund??