When you have a package that was shipped through FedEx and then Transferred to USPS the shipping method is called FedEx Smart Post.
Once FedEx has handed your package off to USPS you go to the USPS tracking website and enter the FedEx tracking number into the USPS tracking information box.
But you also add a 92 in front of the FedEx tracking number.
When you add the 92 to the FedEx tracking number and insert it into the USPS tracking website box and then track it the USPS system should locate your tracking number and you'll be able to track the FedEx Smart Post Package through USPS website.
It can take 24 hours for the USPS to get the tracking information into their system once FedEx hands the package off to them.