There are 6 US Post Offices in Cassia County serving a total of 22,622 residents equating to 3,770 residents per post office. It's estimated that approximately 36,107 packages are handled across the 6 county post offices annually. Cassia County is a county located in the U.S. state of Idaho. As of the 2010 Census the county had a population of 22,952. The county seat and largest city is Burley. Cassia County is included in the Burley, ID Micropolitan Statistical Area."