There are 6 US Post Offices in Colusa County serving a total of 21,078 residents equating to 3,513 residents per post office. It's estimated that approximately 33,643 packages are handled across the 6 county post offices annually. Colusa County is a county located in the U.S. state of California. As of the 2010 census, the population was 21,419. The county seat is Colusa. It is located in the Central Valley of California, northwest of the state capital, Sacramento."