If you are interested in logistics, how mail is delivered is impressive. While most people think Fedex or UPS in the US beat the USPS easily, they don' t know that USPS processes 23+ million mail pieces, letters packages etc., per hour, that's more than Fedex and UPS combined per day.
A reason the USPS system compared to private carriers seem less agile and inefficient is it does more volume than the privates. USPS also has low economy rates that the privates don't [like parcel post, or book rate], yet it still delivers for the same price whether its next city or in the Marianas Islands.
USPS also delivers were Fedex and UPS etc., do not, they cross contract with each other and buy services, USPS buys air space on Fedex, volume trucking with UPS etc., while the privates contract delivery to remote locations with USPS. The USPS tracking number could go haywire, if your package is part of those cross connections to the private systems.