There is a global unified postal service body called the Unive | ELI5

There is a global unified postal service body called the Universal Postal Union . The Union creates a framework and standards so that ALL countries can exchange mail freely without forming individual contracts with every other country.
When one pays for the postage for international mail, a portion of the money goes to the home country's postal service and a portion goes to the destination's postal service. The portion of money exchanged depends on how much mail (in weight) each country is exchanging. This is set up so that the destination country receives money for delivering the postage. Countries that receive more mail annually get less money per kilogram of mail.
The fee paid to the destination county is called a terminal due.