There are 14 US Post Offices in Cass County serving a total of 25,094 residents equating to 1,792 residents per post office. It's estimated that approximately 40,053 packages are handled across the 14 county post offices annually. Cass County is a county in the U.S. state of Nebraska. As of the 2010 census, the population was 25,241. Its county seat is Plattsmouth. The county was formed in 1855 and named after General Lewis Cass. Cass County is included in the Omaha-Council Bluffs, NE-IA Metropolitan Statistical Area. In the Nebraska license plate system, Cass County is represented by the prefix 20 (it had the twentieth-largest number of vehicles registered in the county when the license plate system was established in 1922)."