Buyer Requested a Cancelation

I sold a golf club over the weekend that's boxed up and ready to go to the post office tomorrow (could have gone today but work responsibilities got in the way). The buyer has just requested a cancelation. Am I well within my right to post the item and reject the request? Am I obligated to accept a return once the buyer receives it? I have already received the money into my account minus the 10% promotional fee from eBay and it was an item that was sitting for a while so I'm keen to not have to relist it.