Lost MacBook Air during mail in repair

If Apple sent you a box with a pre-printed return label on it, then Apple is responsible for dealing with FedEx assuming that FedEx at least scanned the package on pickup. Did you happen to make note of the FedEx return tracking number? If you dropped it off at a physical store, then did you get a receipt from the store?

Have you checked with FedEx at the drop off location?

You should contact Apple corporate to see if you have any better luck getting Apple to assist you and to let them know about the issue and your dissatisfaction. You should also check with any local consumer protection laws.

Unfortunately if you don't have a receipt for dropping off the package or a return tracking number to verify at least that FedEx scanned the package, then there is no way to prove you really sent the package. In theory Apple should at least have the FedEx return tracking number which they should be able to provide you so that you can confirm whether FedEx ever scanned the package after you dropped it off. At least with a return tracking number you can attempt to deal with FedEx although I think they will inform you that they can only deal with Apple since the FedEx return label is associated and created by Apple.

Good luck in getting this resolved.