When FedEx gives your package to USPS it means that your package was shipped through FedEx smartpost.
FedEx SmartPost is a cheaper option of shipping packages but is slower than normal FedEx delivery.
First your package travels through FedEx and then once the package reaches your state or local sorting facility for USPS it is then handed off to USPS.
Then USPS does the final delivery of your package.
When USPS gets your package you can continue tracking your FedEx SmartPost package through USPS website by adding 92 to the front of your FedEx tracking number.
Normally FedEx SmartPost packages take 7 to 14 days to reach you.
While normal FedEx and USPS take 3 to 7 days at most to get the package.
If you are waiting on something to be shipped to you through FedEx smartpost you have to be very patient as it will take some time to get to you.
I hate FedEx smartpost and would never use them myself.