USPS is the only carrier that can deliver packages to a PO Box.
Addressing to a PO Box – The type-ahead features don’t recognize PO Boxes. If you are sending to a PO Box, enter the address in your Shipthrifty Address Book first and then pull the address in during label creation.
The PO Box number will go in address line one (example: PO BOX 123).
Using a PO Box as a Return Address – Although you can purchase any carrier label with a PO Box as a return address, we don’t recommend it. If a package is returned for some reason, and it’s with a carrier other than USPS, getting it back may prove challenging and may result in additional surcharges.