It's not really a subsidy so much as an underpayment by the foreign goverments[1]. There is no tax money involved, and all the subsidizing comes from the increased revenue from raising domestic shipping costs.

The UPU has been around for decades, but it's only been since 2010 that the USPS has formalized the process with the "ePacket" shipping.

[1] https://www.washingtonpost.com/news/storyline/wp/2014/09/12/...