The Stamp Problem:

https://en.wikipedia.org/wiki/Postage_stamp#The_nineteenth_century

Before the introduction of postage stamps, mail in the United Kingdom was paid for by the recipient, a system that was associated with an irresolvable problem: the costs of delivering mail were not recoverable by the postal service when recipients were unable or unwilling to pay for delivered items, and senders had no incentive to restrict the number, size, or weight of items sent, whether or not they would ultimately be paid for.