Since 2018 the UPU moved 40 countries, including China, to a new tier and this actually is causing them to already pay higher terminal dues. This decision was made in resolution C 7/2016, see the list of 40 countries in "group III" at page 212 of http://www.upu.int/uploads/tx_sbdownloader/actCompendiumDeci...
The tiers that countries are classified into are called "transitional" (developing country) and "target" (developed countries.) Transitional countries pay lower terminal dues. Target countries pay higher terminal dues. The longer you have been a target country the higher are your terminal dues. And again, China has been a "target" (developed) country since 2018. This is explained in http://www.upu.int/uploads/tx_sbdownloader/questionnaireTdrA...:
Countries in the target tier prior to 2010 pay the most
Countries that entered the target tier between 2010 and 2012 pay less
Countries that entered the target tier after 2016 pay even less
Countries that are still in the transitional tier pay the least
Every few years these dates change (by approving agreements amongst UPU members) and countries pay more and more. So everything is set for China (and 39 other countries) to pay more and more as the years go by, since they've been moved to the target tier in 2018.
This 2015 USPS report explains it (https://www.uspsoig.gov/sites/default/files/document-library...): «Forty of these countries, including all of the BRICS except for India, will join the target tier next year. However, moving these countries to the target category may not immediately lead to significant terminal dues payment increases. The UPU Congress will approve new rates, for the period from 2018 to 2021, next year — meaning implementation is 2 to 6 years after a decision. The new target countries may continue to have an advantage during this period.»
Keep in mind this USPS report was written in 2015. This is now 2019 so the terminal dues have already been increasing 2 years in a row (2018 and 2019) and will continue to increase until 2021: https://www.lexingtoninstitute.org/terminal-dues-time-end-dr...
They increase 13% every year. In 2019 they are up 27.7% since 2017. In 2021 they will have increased 63% since 2017.
So the reality is that the US leaving the UPU is no because China pays low terminal dues. They want a deeper reform that address other issues as stated by the 2015 USPS report: excluding packages (as opposed to letters) from terminal dues, stopping private carriers from being at a disadvantage compared to national posts who receive terminal dues, etc