Montenegro has a long history of existing as a country. The first records of the name Montenegro exist in 1296, when it was called Zeta. The name Montenegro started to be much more common in 1430. Finally, Montenegro regained its independence in 2006.
There is no place called Serbia & Montenegro. Between 1998 and 2006 there was a country called Serbia & Montenegro but since 2006, when Montenegro declared independence, they have been 2 separate countries - Serbia and Montenegro. Postal codes depend on which city you are posting to.