From Dragon Eye Atlas
Couriers deliver messages or packages to recipients both within a city and across larger distances. The Courier Service guarantees delivery by acting as a trustee - the sender will bring his message or package to the nearest Courier Station and sign a Courier Request - offering a payment of his choice for delivery to a specific address. He deposits the payment at the station. Couriers will pick up deliveries from the station. Those with a higher payment will receive priority while lower-paid deliveries often wait around until a courier can bundle them up with several other deliveries to the same or nearby destinations.
Delivery is proven by the use of Courier Tokens, which are available at every Courier Station. Those who expect deliveries will pick up a token at the station, and those who get deliveries regularily may have a number of them ready at all times. The courier will be paid when he brings the token and the Courier Request paper to the nearest Courier Station. If a recipient does not have a delivery token with them, the courier will deliver to the nearest Courier Station and the recipient has to pick up his delivery from there.
This simple system has evolved into a long-distance postal service where couriers often bring a delivery only part of the way, and then sell it off to another courier who will undertake the next leg of the journey. Since deliveries pay a fixed sum, couriers treat request papers and delivery tokens as if they were a form of currency.
Couriers have also begun to specialize - many focus on short-distance deliveries, typically within the same city, as they can make many deliveries during one day. This has brought prices for deliveries within a city down to a few copper pieces. Other couriers have specialized in deliveries between cities, often running (yes, couriers typically move on foot, the job does not pay well enough to do it by horse) back and forth between the same few places. On these routes, however, merchants and other travelling folk are also buying up deliveries in bulk to make a few additional coins. Essentially, a merchant who is anyway bringing wares from city A to city B will also pick up messages and small packages and simply sell them to the local Courier Station at his destination, where city couriers will deliver them to the recipients.
Deliveries within the same city are usually completed within the same day, sometimes within the hour. Across well-travelled roads, service can take longer and a message across the realm will often travel for weeks or months as it is picked up and bought and sold by multiple couriers. Messages do occasionally get lost, as the roads can be dangerous, but overall the Courier Service is quite reliable.
The concept of the Courier Service is heavily inspired by the book The Wandering Inn.