The postal service of the sending country pays to the postal service of the receiving country a fee, called "terminal due", for delivery. There exist also "transit charges" and other charges. This is regulated by the Universal Postal Union.
That is correct, but only when the letter/parcel is entirely handled by the postal services of the various countries involved.
What began this thread,
the Linehaul Office, is about doing things at a scale and at a pre-sort, that the sender can handle it to the last possible USPS injection point before final delivery. That is typically a USPS facility called a
Network Distribution Center (which for my address, the NDC is in Jacksonville FL). The other part of this story (which also bypasses the UPU narative) is that very high volume shippers can get negotiated rates. Those negotiated contract rates are
confidential, only USPS and the shipper know what they are. What is happening here is that the shipper has (almost certainly aggressive) negotiated rates for USPS zones 1 and 2, which should cover all
last mile handling of the items (between the NDC and the destination address). In other words, USPS is not hauling on their long haul transportation network, the shipper is. The (typically small) parcels are very likely being pre-sorted in CN, prior to departure, so that when they land in the US, all that has to be done is a bulk cargo movement, from the port of entry to the destination NDC. USPS accepts the bulk shipment, then dumps it into their sorting system, for final delivery.
I can now report that my first AliExpress order has arrived at
the Linehaul Office