By 1874 the system of international carriage of letters was a mess. Countries had to negotiate postal treaties with each other and most treaties called for cross payments where part of the postage of a letter was remitted by the sending post office to the receiving post office. This was confusing and difficult to keep tabs of but of further complexity was the cross payments to transit countries (say when a letter from the United States landed in England, was sent to Poland and then across land to Russia). Nearly thirty five years after the Penny Black, with world commerce rising, the system of cross payments just wasn't working. It was slowing down communication and as postage rates dropped it was becoming more expensive to account for the letters than to carry them. In 1874 the United States proposed an international congress to discuss the problem and what emerged was an agreement for an organization that was one of the first international cooperation organizations (it was in fact the third) and certainly the most successful in realizing its goal-simplifying and lessening the cost of postal communication. The Universal Postal Union established three things. First, international letter rates could be set by the sending country but they had to be the same anywhere in the world. Second, countries had to treat incoming mail the same as domestic mail in terms of delivery-no favorites. And third, the system of international credits and debits was abolished. No more accountants eating up the lions share of postage costs (This last part has been amended a bit over the years to require some cross payments as some smaller countries have lowered their international postage rates so as to become international mail hubs and mailing centers). Due to the UPU, international communications boomed throughout the late nineteenth and early twentieth centuries and cooperation in the UPU helped lead to the ideas for the League of Nations and the UN. In a form of self congratulation, Postal Agencies have long commemorated every anniversary of the UPU and today UPU thematics are very popular sharing with Olympic thematics the honor of having some of the most popular and scarce sets as part of its heritage.
Universal Postal Union