I've never thought about how the postal delivery systems work.
How do RM get paid for delivering mail from other countries?
Wikipedia has a bunch of info on it at
https://en.wikipedia.org/wiki/Universal_Postal_Union#Terminal_dues . A quick summary is.
Originally the Universal postal Union worked on a settlement-free basis. Countries were responsible for delivering mail handed to them by other countries without compensation. The idea was that this would be reasonably fair since the flow of letters would be reasonably symmetric.
However, later it became clear that developing countries received far more mail than they sent. In 1969 "terminal dues" were introduced to compensate countries which received more mail than they sent. The system of terminal dues was tweaked many times In 1999, a system was put in place that essentially gave developing countries a discount on deliveries to developed countries. Even as recently as 2010, the US was quite happy with the system, since it let the USPS ship mail to many countries far cheaper than private carriers could achieve.
However, then we got the rise of mass "direct from china" shipping, driven by sites like amazon, eBay and aliexpress. The USPS went from making a surplus on international mail to making a deficit in 2015 and trump was not happy with what he saw as the US government subsidising the delivery of cheap crap from china. Trump threatened to withdraw from the UPU.
A couple of years later, a compromise was negotiated. UPU terminal dues were raised, and countries were given the option to self-declare terminal dues, based on domestic postal rates for goods and some documents (but not ordinary letters). The wikipedia article doesn't say what countries actually adopted said self-declared rates, but toing some searching finds
https://digitalsociety.eui.eu/wp-content/uploads/2023/06/6A_Campbell.pdf which says that all but two "economically advantaged countries" have adopted self-delcared rates.
So in summary it seems like the west was subsidising packages from China in the mid-late 2010s and early 2020s, but such subsidy either has been or is in the process of being phased out.