UPS provides tracking numbers that help ensure the safe arrival of your package. Packages with tracking numbers are scanned at each checkpoint on their journey, allowing the packages to be tracked as they move through the UPS system to their destination. If an issue arises with a package the location is readily identified and makes the chances of recovery more likely. Also, UPS has a Customer Service person available to further assist in resolving any issues with your shipment.
USPS only offers Delivery Confirmation numbers. Delivery Confirmation gives the date, zip code and the time the package was delivered. If delivery was attempted, the date and time of attempted delivery will be given. There is no capability to determine where your package is at a given time in its movement to you. Packages with Delivery Confirmation do not get scanned at each checkpoint on their journey like a tracking number, making it impossible to locate a package that may have gone astray.