Krell_man, I think Fed Ex remains responsible to the shipper for any damage that occurs in transit. The shipper's contract is with Fed Ex, not the USPS. The USPS is only acting as an agent of Fed Ex., which remains liable. Fed Ex may be able to collect from the USPS for the amount Fed Ex ends up paying its customer for damage caused by the USPS, but the customer is not required to deal with the USPS.
My reasoning assumes that Fed Ex doesn't insert a clause in the terms printed on its shipping slip that shifts liability to the USPS for damage the USPS causes. That would be a disaster for the customer, because each carrier would point the finger at the other one, and the customer would have no way to determine which one caused the damage. If dealing with either one of these carriers alone is brutal, imagine what a three-party merry-go-round would be like. However, I think the customer backlash would be so bad that Fed Ex wouldn't try such a maneuver.
My reasoning assumes that Fed Ex doesn't insert a clause in the terms printed on its shipping slip that shifts liability to the USPS for damage the USPS causes. That would be a disaster for the customer, because each carrier would point the finger at the other one, and the customer would have no way to determine which one caused the damage. If dealing with either one of these carriers alone is brutal, imagine what a three-party merry-go-round would be like. However, I think the customer backlash would be so bad that Fed Ex wouldn't try such a maneuver.