Yes, it is true that in the early 15th century when the novel is set, the Teutonic Order had a sort of a postal service functioning within the borders of the Teutonic State. The Polish word used there is 'poczta' which means 'post' or rather, in this context, 'postal services'. In my opinion English expression 'post' used in the translation is closer to the mark than Chinese yizhan beucase the latter, much like the already mentioned mansio or shukuba were open establishments that catered to various people, including state officials, private messengers and other travelers, while the post of the Teutonic Order was a part of the state administration, and thus the postal facilities were located within the castles and were accessible only to the members and servants of the Order who carried out their duties.

I would also like to point out that the postal services in 'medieval' Europe were rare or even non-existent chiefly not because people were illiterate, but rather because the decentralized nature of the countries resulting from the feudal relations made it hard to introduce any sort of an uniform service within the borders of, say, a single kingdom. Please note that people who were illiterate were largely living most of their lives in a very small area and thus had little use for a correspondence, while those who could have benefited from the circulation of written letters, such as diplomats, officials (both lay and ecclesiastical), courtiers, bankers or merchants were usually literate even if only in their vernacular language. The early forms of postal services were introduced in mid-13th century in France, and until 14th century there were similar systems employed by various local rulers within their respective lands. Philip IV took all the couriers in France under royal protection in 1297, contributing to the development of the early postal system that were initially existing as various local services, not yet coordinated within a single system. Throughout 14th century, similar postal systems started to develop in cities, especially in the important trading hubs that maintained complex relationships with other cities and rulers, or the cities that were organized into alliances (e.g. Lusatian League, Rhenish League of Towns) and thus required more or less reliable way of document delivery. Initially, these services were pretty small in size, with the 14th-century ledgers of the city council in Hamburg stating that it had only two postal couriers on the council's payroll, what has been increased to four in 1534. It is also worth noting that in the second half of the 14th century, postal services were already well-developed in many major Hanseatic cities, especially in Gdańsk (Danzig) that was an important trading hub and an communication and trade intermediary between the Hanse, Teutonic Order, Poland and Livonia.

Of note is also a semi-formal institution that existed in some German cities and was known as 'Metzgerpost' (lit. 'butcher post') centered on the butchers who sourced their meat outside of the city and thus quite commonly travelled between towns and were sometimes employed as couriers by the local organizations and city officials. Some of the 'butcher post offices' remained until 18th century, when they were included in the structure of the local Postal Services.

Now, let's return to the topic at hand. The postal service of the Teutonic Order has been established sometime in the late 13th century, with the year 1276 often being quoted as the earliest attested date of the existence of a courier system within the Teutonic state. Couriers were usually recruited from among the local Prussian population that were introduced into the structures of the Order as lay servants, usually delegated to perform menial tasks and assist the knights. They were remunerated for their services and, depending on the local customs and agreements, they might have been receiving a fixed annual salary or a payment for each individual travel, usually proportional to a distance covered and importance of the route, as existing ledgers suggest that couriers sent to major seats of the Order, such as Marienburg (now Malbork), Koenigsberg (now Kaliningrad) or Thorn (now Toruń) were receiving higher fees than those sent on the same route to smaller cities, like Brauensberg (now Braniewo). The position of a postal courier was a dedicated one, meaning that the service was provided by specially selected servants trusted with letter delivery, hence the name 'bryffjunge' ('Briefjunge' in modern German), lit. 'letter boys'. In the contemporary descriptions they are mentioned as wearing distinctive blue clothing, most likely a blue cloak or tunic worn over the regular garb.

Some of the letters were also being given priority over others, as evidenced by the annotations on the outside of the documents, such as ane sumen (without delay, ohne säumen in modern German), ane alles sumen (without any delay), tag und nacht ane alles sumen (day and night without any delay) or tag und nacht ane alles sumen macht leith doran (day and night, without any delay, go directly). In addition, the annotations made on the letters that were transferred to the following couriers sometimes allow to pinpoint the time of the delivery with an accuracy of a single hour. Such information allows to estimate an average time of mail transport to 4-5 km/h (2.5-3 mph) which is at least four times higher than the average speed of an overland travel in the corresponding period and place (this is the average speed of the entire travel, not just the average speed of a moving courier). Please note however, that these averages are typical for relatively short distances within the Teutonic State, where couriers had an ability to change horses or transfer the letters to another courier who covered the subsequent stage of travel. In case of long-distance correspondence, such as one sent from Basel or Geneva to Marienburg or from Vysehrad to Thorn, the average speed is much more similar to the general average of travel equal to 25-30 km (15-18 miles) per day.

Postal service was based on the network of castles, usually local commandories of the Order proper, or the seats of lay power within the Teutonic states. Such places usually had a dedicated postal room referred to as 'bryffstall' ('Briefstelle' in modern German, lit. 'letter station'), where the incoming correspondence was received, registered and transferred to other couriers if necessary.

This quite rudimentary postal service quickly emerged in Teutonic State chiefly due to the character of the Order and its lands. Unlike many of the European kingdoms in 13th and 14th century that were often highly decentralized and fragmented, Teutonic State was governed by Teutonic Order that was a strictly hierarchic organization, what allowed high level of coordination between the constituent parts of the quickly expanding network of castles and associated lands. To facilitate such coordination, it was necessary for the members of the orders, especially the leaders of local commandories to exchange information, what eventually led to the development of the courier network and then the dedicated postal service. This was especially important because the Grandmaster, along and the highest-ranking Order members, although nominally residing in Marienburg, usually traveled around the country in spring and autumn, visiting other major cities and administrative centres, while at the same time requiring information about the urgent matters should something happened in another part of the Teutonic State. For example, Konrad von Jungingen, Grandmaster in the times the first part of Knights of the Cross takes place (elected in 1393, died in 1407) has visited various places in the Teutonic states more than 800 times (not counting returns to Marienburg), meaning 60 visits per year, a quite intensive itinerary even for modern standards.

Thus, in the times the novel has been set (1399-1410), the Teutonic State already had a well-developed postal services that catered to the administrative needs of the quickly developing polity, but contrary to author's allegations, it wasn't unknown elsewhere, although a unique character of the Teutonic State facilitated the emergence of such centralized and state-wide services.

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Jahnke, J., Zimmermann, H., Die Postwege des Deutschen Ordens in der ersten Hälfte des 15 Jahrhunderts [Postal services of the Teutonic Order in the 1st half of the 15th century], [in:] Mortensen, H., Mortensen, G., Wenskus, R. (Eds.), Historisch-Georaphischer Atlas des Preussenlandes [Historical and Geographical Atlas of the Prussian Lands], Wiesbaden 1967.

Boockmann, H., Die Briefe des Deutschordenhochmeisters [Letters of the Teutonic Order Grandmasters], in: Schöningh, F., (Ed.), Kommunikationspraxis und Korrespondenzwesen im Mittelalter und in der Renaissance [Communication and Correspondence Services in the Middle Ages and Rennaissance], Paderborn 1998.