There's four definitions of United States

1. USA, The nation, The national government (DC + 50 States)
2. DC, The DC Municipal Government, the Federal Zone
3. The 50 States.
4. A corporation

United States is often misleadingly referencing 2. DC. So, a domestic mail, with zip codes, is between federal government agencies. A non-domestic mail is between the States/Republics/Nations which are the 50 States. California is a State. STATE OF CALIFORNIA is a corporation much like United States can be a corporation. You can find it on a Dunn and Bradstreet website and it'll list government employees you know as the directors/officers/agents.

So, not quite.

What you're doing is sending mail from one of the states of the union to another state of the union and that is considered foreign with respect to the United States** which in this case is referencing the Federal Zone or the Federal Enclaves ie DC. It's not foreign to the United States* or the United States*** but it is foreign to the United States** and that's why it's non-domestic.