Personally I would apologise for the missing item, and for them to open a case for item not as described.
Once opend you need to accept the case within 3 days, and refund, if you wish for the item to be returned first, you pay for that fully tracked return postage and refund onnly when that item is back with.
Stick within those timelines, your seller fees are refuded, and your seller performance not affected.