There are 10 US Post Offices in Flathead County serving a total of 133,379 residents equating to 13,338 residents per post office. It's estimated that approximately 212,888 packages are handled across the 10 county post offices annually. Flathead County is a county located in the U.S. state of Montana. As of the 2010 census, the population was 90,928, making it the fourth-most populous county in Montana. Its county seat is Kalispell. The numerical designation for Flathead County (used in the issuance of license plates) is 7. It is south from the Canada\u2013US border of British Columbia. Flathead County comprises the Kalispell, Montana Micropolitan Statistical Area."