The letter below proves that Washington (state) is not part of United States. It was sent with just four cents of postage (two two-cent stamps) and clearly marked as non-domestic (to U.S.) and without United States.
Washington is a nation-state. The USofA is an alliance of 50 nation-states. U.S. is the entity that manages the administrative affairs of USofA and manages the territories. Domestic, with respect to U.S. means Puerto Rico, DC, etc.