There are 7 US Post Offices in Frederick County serving a total of 34,405 residents equating to 4,915 residents per post office. It's estimated that approximately 54,914 packages are handled across the 7 county post offices annually. Frederick County is a county located in the Commonwealth of Virginia. As of the 2010 census, the population was 78,305. Its county seat is Winchester. The county was formed in 1743 by the splitting of Orange County. It is Virginia's northernmost county. Frederick County is included in the Winchester, VA-WV Metropolitan Statistical Area, which is also included in the Washington-Baltimore-Northern Virginia, DC-MD-VA-WV-PA Combined Statistical Area."