There are 6 US Post Offices in Grant County serving a total of 7,108 residents equating to 1,185 residents per post office. It's estimated that approximately 11,345 packages are handled across the 6 county post offices annually. Grant County is a county located in the U.S. state of South Dakota. As of the 2010 census, the population was 7,356. The county seat is Milbank. The county was founded in 1873 and organized in 1878. It is named for Ulysses S. Grant, 18th President of the United States."