There are 21 US Post Offices in Charleston County serving a total of 341,090 residents equating to 16,242 residents per post office. It's estimated that approximately 544,418 packages are handled across the 21 county post offices annually. Charleston County is a county located in the U.S. state of South Carolina. As of the 2010 census, its population was 350,209, making it the third most populous county in South Carolina (behind Greenville and Richland Counties). Its county seat is Charleston. The county was created in 1901 by an act of the South Carolina State Legislature. Charleston County is included in the Charleston-North Charleston, SC Metropolitan Statistical Area."