A goober is speaking to her psychiatrist.

"I'm on the road a lot, and my clients are complaining that they can never reach me."

"Don't you have a phone in your car?"

"That was a little too expensive, so I did the next best thing.  I put a mailbox in my car."

"Uh.  How's that working?"

"Actually, I haven't gotten any letters yet."

"And why do you think that is?"

"I figure its because when I'm driving around, my zip code keeps changing."