UPS SurePost

UPS SurePost is an economy service that combines the consistency and reliability of the UPS Ground® network with final delivery provided by the U.S. Postal Service to your residence, business, PO Box, APO or FPO.

As you can see in the screenshot of this sample shipment the package was delivered by UPS to the local Post Office and then USPS delivered it to the final destination. Not only does UPS provide a tracking number, it also provides a USPS tracking number in the "Additional Information" section.