So annoying when a buyer requests this After purchase when the time to do so is before , to avoid all of this hassle.
Send them a message that to do this, you face a late delivery defect so the best way forward for you both is to cancel the order, with the buyer's acceptance, this refunds the buyer , and then to relist it when the buyer is back for them to buy and pay again from your eBay listing.
Cancel the order citing Buyer Requests to Cancel, this refunds them, and your seller performance is not affected.
@concord310169