I would even go further and suggest at this point opening a dispute with ebay is not possible as they have already sent the item back outside of the MBG process. I'm 99% confident it's not retrospective, but pretending it is (or that I am mistaken), you would NOT be liable for the return post as an amount was not agreed before the item was sent. There would be no way for ebay to enforce you to pay anything more than what the buyer originally paid.
Paypal are also strict about returns being conducted within their returns process in order to be eligible for buyer protection, but in the very unlikely event the buyer got lucky with a rep, again they cannot make you pay more than the initial transaction amount.
So I would go ahead and refund only the item price, then ask ebay for a fvf fee credit on the amount (they calculate item price and postage fee seperately if you have listed it that way) - they will be able to see that a refund for $xx.xx occurred. You don't require a case to be opened. However if you deduct any other costs you feel you incurred then you get your FVF back.
