At least in our rural WV town, USPS will not accept delivery of UPS or FedEx packages to their P.O. Boxes.
I thought a P.O.Box was a great solution and was going to rent one, but if I can only get USPS packages, I then the rental cost is too high to justify.
I have a roadside mailbox at my house and USPS puts the UPS/FexEx packages in the mailbox, or leaves them on a porch if too large for the mailbox.
Have never asked if they hold such packages at the PO for those with boxes there, but it would seem they should since they deliver them out on the RFD routes.