"The new 1969 agreement included terminal dues, the receiving postal agencies would charge the country of origin for delivering the post on by-weight basis.
To be blunt, shipping from China to USA (and most of the rest of the world) is much cheaper than shipping domestically in USA because the terminal-dues are much too low and account only for the total weight of the post, not for the number of items.
For example, in 2013 China paid $1/kg (or $0.50/lbs)[1] to have post delivered in USA. This is MUCH lower than domestic pricing, especially if it’s light-weight objects.
As an example, I just ordered a new wristband for a watch I own from China. I just weighed it, and it weighs 78 grams. That means China would pay less than 8 cents to have this wristband delivered anywhere in USA.
By comparison, sending the same weight in a domestic letter in USA[2] would cost 92 cents, or a factor of 11.5 more.
Reality is that this agreement now acts as a massive subsidy to especially China who produces a lot of low-cost low-weight products and can now deliver them to customers worldwide for a price much lower than local companies can.
This situation will persist until the relevant agreements are renegotiated. I’m frankly surprised that hasn’t happened already.
[1] https://archive.is/20121212225536/http://www.upu.int/en/acti...
[2] https://www.stamps.com/usps/postage-rate-increase/
https://www.quora.com/How-is-China-able-to-offer-free-worldw...