On the other hand, there could be a central distribution point in each town. A new NPC, perhaps. Or could be incorporated into the vault system across Istaria.
Walk up, greet NPC, fill out message, give coin and the message is there at THAT town.
Now they need people to come by to pick up big packages of messages going to another town. Who isn't making the run from Dalimond to Kirasanct by way of Acul nowadays and could drop off the package at Acul from Dalimond? So, some brave adventuring type comes up and volunteers to collect a profit from all the fees collected for all the messages going to Acul from Dalimond and makes the run, delivering them safely and getting paid. Oh, hey. There's a set going from Acul to Kirasanct.
Repeat the process any dozen times daily depending on who's sending messages where. Of course, longer routes mean more message fees and a higher collection, unless they care to get bounced around the system from time to time at a hub like Heather, Bristugo, or Tazoon until they can get to the outer limits of everywhere.
Might even make up the beginnings of a class that gains experience by crossing all of Istaria to send messages all over the place. Runner?
Hmmm, single runners aren't always safe especially if they have to do that Lerena to Morning Light overland. Get a group and they can haul trade goods too at the same cost? What do you know, you now have a second profession: Caravan hauler.