[acdha]

It's hard to understate just how important the postal service was to the country: beyond driving so much commerce (and hence making mail deliveries a valuable target) it was used for all kinds of official documents and financial documents (money orders were introduced to reduce the risk further). Travel being expensive and hard, no alternatives like phone or email, etc. means that you send letters a LOT and things like postmarks or registered mail signatures have a substantial legal weight. Making a highly trusted service with special status to deter thieves available to everyone at a modest fee was a really smart move.