Now, the cost of running the postal service is the cost of supporting the whole network of delivery vans, sorting offices, collections and so on, including all the staff.
The cost of supporting the network is the same whether it carries a very large number of packets or zero packets (up to the point where you have to add infrastructure to cope with extra traffic. Yes, like the Internet).
This economic structure means you can carry traffic at a marginal cost if you know the cost of supporting the whole network is covered.
All of which was worked out before Sir Rowland Hill launched the Uniform Penny Post in the UK in 1840. This disrupted the whole messenger business (where you paid for distance traveled) and was widely copied everywhere else. After that, nations formed a Universal Postal Union on the basis that "we'll deliver your letters if you deliver ours" (like the Internet).
In the early days of the public Internet (early 1980s), I used to explain how it worked by comparing it to the penny post. It's nice to be able to do the reverse ;-)