There are 3 US Post Offices in Musselshell County serving a total of 4,601 residents equating to 1,534 residents per post office. It's estimated that approximately 7,344 packages are handled across the 3 county post offices annually. Musselshell County is a county located in the U.S. state of Montana. As of the 2010 census, the population was 4,538. Its county seat is Roundup."