Some information about the UPU from Wikipedia:
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.
Prior to the establishment of the UPU, a country had to conclude a separate postal treaty with each other country that it wished to carry international mail to or from. The United States called for an international postal congress, which was held in 1863. This led Heinrich von Stephan, Royal Prussian and later German Minister for Posts, to found the Universal Postal Union, the second oldest international organization (after the ITU). It was created in 1874, under the name "General Postal Union", as a result of the Treaty of Berne signed on 9 October 1874. In 1878, the name was changed to "Universal Postal Union".
The UPU established that
- there should be a more or less uniform flat rate to mail a letter anywhere in the world;
- postal authorities should give equal treatment to foreign and domestic mail; and
- 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.