The time to make these requests are before the buyer purchases and pays.
You are not obligated to send by Fedex.
If it were me, I would tell the buyer that it is not possible. For me, shipping Fedex would mean an extra trip to town versus the USPS carrier picking up from me. That, in my opinion is a deal breaker.
You can cancel the label and try to get the difference in shipping cost/amount from the buyer but it seems like a lot of trouble to go out of your way to accommodate the request. You still need to meet your shipping time expectation to eBay.
