There are 3 US Post Offices in Sanborn County serving a total of 2,604 residents equating to 868 residents per post office. It's estimated that approximately 4,156 packages are handled across the 3 county post offices annually. Sanborn County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 2,355. Its county seat is Woonsocket."