Oh, the terminal dues issue.[1] International mail is delivered in the destination country but paid for in the sending country. The UPU has rates for this, and they vary with the development category of the sending country. The problem is that China is still getting the rate for underdeveloped countries. In 2021, that's scheduled to change. Every 10 years this gets looked at.
[1] https://www.lexingtoninstitute.org/terminal-dues-time-end-dr...