There are 5 US Post Offices in Custer County serving a total of 8,139 residents equating to 1,628 residents per post office. It's estimated that approximately 12,991 packages are handled across the 5 county post offices annually. Custer County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 8,216. Its county seat is Custer. The county was created in 1875 and organized in 1877. Custer County is included in the Rapid City, SD Metropolitan Statistical Area."