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.
@shapr I've been enjoying reading the Durants' "Rousseau and Revolution"; evidently this was the case in France at the time too, and at times Rousseau couldn't afford to receive his mail.