The mission of a postal service is to deliver and collect mail over all its attributed territory. But should it do so at a uniform cost to users?
Delivering mail in rural areas is much more costly than in urban ones. Yet in most countries, postal fee do not differentiate delivery areas, in fact they do not even distinguish distances, as long as delivery stays within the country. In economic terms, this is highly inefficient, as areas where delivery is easy subsidize areas where it is difficult. This encourages rurals to send and receive more letters than they should (for efficiency's sake).
Of course, the free market could circumvent that by offering at a lower cost delivery services in urban areas. This would erode the principle of uniform cost, and is thus banned in many countries by granting monopoly for postal services. This has been gradually relaxed, in particular for packages.
But why have this principle of uniform cost? Why encourage people living in remote places that are economically not self-sustaining? Is having economic development on all the territory a public good?