In the old days, the postal practices required that senders pay for the sending and receiving post offices' fees. This was revised in modern times so the sender only pays the charge of the sending post office. Thus, because China does not charge its own residents very much for postage, Chinese sellers don't pay much to send parcels to the USA. This is a form of a Chinese government subsidy that enables its merchants to compete (and compete unfairly) - China doesn't have to break even when running its post office.

The nations calculate the total shipments between countries, and there are fees paid from government to government, called "terminal dues." Because this is from government to government (rather than assessed against shippers), the Chinese government can absorb the payments it makes to the USA, Canada, etc., allowing its merchants to reap a benefit.

This article doesn't explain it in full, but discusses some of this. https://en.wikipedia.org/wiki/Universal_Postal_Union