Who's paying?

Fuad asked:

How is a nation paid for sending and receiving foreign mail?

Swampgrowth answered:

The postal service of each nation is responsible to pay for the delivery of a letter/package to the target nation's frontier (e.g. airport). Secondly, each nation's postal service is responsible to collect letters/packages from all post boxes, post offices, frontier points, etc. Finally, it is also responsible to deliver all letters/packages destined for a point within its borders, regardless of the collection point (which may be a frontier point). In other words, postal services do not get reimbursed by other countries. They must subsidize their responsibilities themselves (i.e. primarily stamp sales).