During the war (or part on the perpetual war)  between Zlobenia and Borogravia described in The Monstrous Regiment,  Ankh Morpork military under Lord Rust and Watchmen under Samuel Vimes were sent in by Vetinari to try and broker an end to the conflict - for  the good of Ankh-Morpork of course. Those in uniform will want to write home, let family know they are safe and well, and to boast of their exploits to mates back home. Thus a postal service is required, and supplies of stamps and postcards are needed.

The battle sites along the Kneck River (from The Compleate Disworld Atlas)

A special postcard was available to the men with a list of options on the message side for the men to tick so as to  compose a message home that satisfied the censors. There are strict rules regarding how much (or little) information is given out in case the mail sack fell into Borogravian hands. One the front there was a 1  zloty Zlobenian stamp and a pre-printed Ankh-Morpork Post Paid stamp (unique amongst Discworld Stamp Collecting) to get the mail out of Zlobenia and to the addressee in the city. The AM stamp is mysteriously  cancelled FIELD POST OFFICE - WOOLPIT SUFFOLK. The 1z stamp has the  rather ornate Zlobenian cancellation.