Only FedEx SmartPost, which uses the USPS system in the last leg of delivery, will deliver to domestic P.O. boxes.
FedEx Express can ship to a P.O. box in some international destinations, including Puerto Rico, but you are required to provide a valid telephone, fax, or telex number.
All other FedEx services do not deliver to PO Boxes for addresses in the United States. If you are attempting to ship to a PO Box using a FedEx service that does not deliver to that address, you will see a warning in the app.
On the ORDERS page:
On the READY TO SHIP page: