Universal Postal Union.

The Universal Postal Union (UPU, French: Union postale universelle) is an international organization that coordinates postal policies between member nations, and hence the world-wide postal system. Each member country agrees to the same set of terms for conducting international postal duties. Universal Postal Union's headquarters are located in Berne, Switzerland

The UPU established that
1. There should be a more or less uniform flat rate to mail a letter anywhere in the world;
2. Postal authorities should give equal treatment to foreign and domestic mail; and
3. Each country should retain all monies it collected for international postage.

One of the most important results of the UPU treaty was that it ceased to be necessary, as it often had been previously, to affix the stamps of any country through which one's letter or package would pass in transit; the UPU provides that stamps of member nations are accepted for the whole international route.
After the foundation of the United Nations, the UPU became its specialized agency.
In 1969 the UPU introduced a new system of payment by which fees were payable between countries according to the difference in the total weight of mail between the respective countries. These fees were called terminal dues. The new system was fairer when traffic was heavier in one direction than the other. As this affected the cost of the delivery of periodicals, the UPU devised a new "threshold" system, which was implemented in 1991.
The system sets separate letter and periodical rates for countries which receive at least 150 tonnes of mail annually. For countries with less mail, the original flat rate has been maintained. The United States has negotiated a separate terminal dues formula with thirteen European countries that includes a rate per piece plus a rate per kilogram, and has a similar arrangement with Canada.