How does mail travel?

Question

If I mail first class from Minnesota to California, what route would it take? Does it always fly? Is the transportation outsourced, and if so, which legs?

Answer 1

Each cluster of zip codes with the same first 3 numbers has a main post office.  For example, if you send it from 95234 to 54325, it will be sent to the 952 post office by car, from which it will go to the 543 post office by plane (usually), and then to the 54325 by car again.  It may or may not be outsourced, it all depends.