I found the reference to the Universal Postal Union interesting and intriguing (because I learnt about it in college). From Wikipedia (https://en.wikipedia.org/wiki/Universal_Postal_Union)
Some mail tracking services leverage the reputation of the UPU to offer their services:
https://www.4tracking.net/en/carriers/upu
https://www.ship24.com/couriers/upu-tracking
I doubt, though, that the UPU is directly involved with these companies, except that the tracking numbering system appears to be the result of a cooperative effort among UPU member countries, so that an item has the same tracking number on its entire journey, which is a good thing.