There are 5 US Post Offices in Boundary County serving a total of 10,943 residents equating to 2,189 residents per post office. It's estimated that approximately 17,466 packages are handled across the 5 county post offices annually. Boundary County is a county located in the U.S. state of Idaho. As of the 2010 census, the population was 10,972. The county seat and largest city is Bonners Ferry. Boundary County was created by the Idaho Legislature on January 23, 1915. It is so named because it borders Canada, the only county in Idaho with an international border."