The central post office on the corner of Sukhbaatar square and Peace avenue (entrance on Peace Avenue) is the best place to go, they are friendly, speak English and are happy to help ( a rare combination in Mongolia).

 

They have a wide variety of postcards on offer there as well as a collection of colourful stamps to send your postcards to your loved ones at home.  Stamps range from about 550 T for postcards, and little more for letters, due to the size. 

 

The prices for the postal services vary but are always reasonable and the service itself is excellent if a bit slow. Letters to the World will ususally take two weeks or so to arrive, it's not very quick, but two weeks is very good considering the size of the Mongolian Post Service.

 

It is worth being cautious in the street immediately infront of the post-office as there are sometimes pick-pockets in the area.