There are 7 US Post Offices in Stillwater County serving a total of 9,481 residents equating to 1,354 residents per post office. It's estimated that approximately 15,133 packages are handled across the 7 county post offices annually. Stillwater County is a county located in the U.S. state of Montana. As of the 2010 census, the population was 9,117. Its county seat is Columbus."