This is a boring but actually really important topic. The Universal Postal Union (UPU) is a global organization of 192 member countries. It does a lot of things but the most crucial thing it does is set the rates at which each nation can charge other nations for delivering their post.
There have been long time complaints that China gets an unfair advantage - witness the flood of Chinese e-packers we deliver every week - but the US government is threatening to pull out of the UPU unless it gets to set its own rates. Anyway, the article from Time Magazine linked to explains it in full:https://time.com/5680413/trump-universal-postal-union/