There are 4 US Post Offices in Carter County serving a total of 1,126 residents equating to 282 residents per post office. It's estimated that approximately 1,797 packages are handled across the 4 county post offices annually. Carter County is a county located in the U.S. state of Montana. As of the 2010 census, the population was 1,160, making it the fifth-least populous county in Montana. The county seat is Ekalaka.