There are 3 US Post Offices in Custer County serving a total of 11,796 residents equating to 3,932 residents per post office. It's estimated that approximately 18,828 packages are handled across the 3 county post offices annually. Custer County is a county located in the U.S. state of Montana. As of the 2010 census, the population was 11,699. Its county seat is Miles City. The county was formed in 1877 and named in honor of George Armstrong Custer."