There are 6 US Post Offices in Columbia County serving a total of 45,644 residents equating to 7,607 residents per post office. It's estimated that approximately 72,853 packages are handled across the 6 county post offices annually. Columbia County is a county located in the U.S. state of Oregon. As of the 2010 census, the population was 49,351. The county seat is St. Helens. It was named for the Columbia River, which forms its eastern and northern borders. Columbia County is included in the Portland-Vancouver-Hillsboro, OR-WA Portland metropolitan area."