Why did they ask to cancel? A lot don't bother with cancellations these days because it gives the buyer the power. Instead, use the unpaid item dispute. Send the buyers a message saying a request will be opened 4 days after purchase. Tell them they don't need to do anything and ignore any automated requests to pay. 4 days later you will close the request and it will all be taken care of.
By doing that, the buyer can't leave you feedback. If they do decide to leave bad feedback while the dispute is open, eBay will remove it once the dispute is closed.
