There are 13 US Post Offices in Shiawassee County serving a total of 75,396 residents equating to 5,800 residents per post office. It's estimated that approximately 120,340 packages are handled across the 13 county post offices annually. Shiawassee /\u02cc\u0283a\u026a.\u0259\u02c8w\u0252si\u02d0/ is a county located in the U.S. state of Michigan. As of the 2010 census, the population was 70,648. The county seat is Corunna. The county was founded in 1822 and organized in 1837. Shiawassee County is named for the Shiawassee River, and means 'river that twists about'. In 2010, the center of population of Michigan was located in Shiawassee County, in the township of Bennington. Shiawassee County comprises the Owosso, MI Micropolitan Statistical Area and is included in the Lansing-East Lansing-Owosso, MI Combined Statistical Area. The Shiawassee County Courthouse was designed by Claire Allen, a prominent southern Michigan architect."