There are 4 US Post Offices in Custer County serving a total of 3,929 residents equating to 982 residents per post office. It's estimated that approximately 6,271 packages are handled across the 4 county post offices annually. Custer County is a rural mountain county located in the center of the U.S. state of Idaho. As of the 2010 census, the population was 4,368. The county seat is Challis. Established in 1881, the county was named for the General Custer Mine, where gold was discovered five years earlier. Custer County relies on ranching, mining, and tourism as its main resources."