Why does each country usually keep the postage it collects for international letters?

Under the rules established by the Universal Postal Union, each country retains the postage it collects for international mail to minimize the need for complex international accounting.1

This principle, in place since 1875, was originally justified on the assumption that international correspondence is reciprocal—meaning that for every letter sent, a reply is typically generated, balancing the workload between the involved postal administrations. Under this system, the country of origin keeps the revenue, and the country of destination provides delivery services without receiving a direct payment for that specific item.1

While this framework simplifies operations, it can create disadvantages for countries experiencing a significant imbalance between incoming and outgoing mail. To address such disparities, the 1969 Congress of Tokyo introduced provisions for compensatory payments in cases of excessive mail volume imbalances.1