Postal services are national, but mail is not. A parcel handed to a US post office is delivered by a foreign administration, and the arrangement that makes this routine is old and unusual.
A single postal territory by agreement
Member countries of the international postal system agree to treat their territories as a single space for the exchange of mail, accepting one another's items for delivery.
That is why domestic postage suffices for an international letter. The sender pays the origin country, and the destination country delivers without collecting again from the recipient.
The agreement also standardizes formats, address conventions and the documents that accompany parcels, so that sorting equipment abroad can process foreign items.
Terminal dues settle the imbalance
Delivering mail costs money, so the destination administration is compensated by the origin administration through payments known as terminal dues.
Rates are set collectively and vary by country group and item type, calculated on weights and volumes exchanged rather than item by item.
Where flows are unequal, the country receiving more mail than it sends is a net recipient of these payments, which is a persistent point of negotiation.
Online shopping changed the balance
The system was designed around letters. The growth of small packets ordered online shifted volume toward lightweight goods traveling one direction in large quantities.
Destination posts argued that dues set for letter mail did not cover the cost of delivering parcels, and that the shortfall effectively subsidized foreign sellers.
Successive rate reforms have moved toward letting countries set their own charges for inbound packets, which narrows the gap with commercial carriers.
Customs runs on a parallel track
Postal items containing goods carry declarations describing contents and value, and the destination country's customs authority decides admissibility and duty.
Because postal channels historically carried low-value items, many countries set thresholds below which duty is not collected, and those thresholds differ widely.
When a parcel is held, the postal service is acting as an agent for customs rather than as a carrier, which is why tracking often stalls at that stage.
Transport is bought, not owned
Postal administrations rarely fly their own international mail. Space is purchased on commercial aircraft, and capacity depends on ordinary passenger and freight schedules.
Disruptions to aviation therefore affect mail directly, and routes with fewer flights see longer and less predictable transit times.
The visible service is a national one, but the underlying network is assembled from contracts with airlines, handlers and other countries' post offices.