There are 9 US Post Offices in Sullivan County serving a total of 21,618 residents equating to 2,402 residents per post office. It's estimated that approximately 34,505 packages are handled across the 9 county post offices annually. Sullivan County is a county located in the U.S. state of Indiana, and determined by the U.S. Census Bureau to include the mean center of U.S. population in 1940. As of 2010, the population was 21,475. The county seat is Sullivan. Sullivan County is included in the Terre Haute, Indiana, metropolitan statistical area."