>Which is getting to the point of my question. Thank you, Jo. Now, would you say "just stay in the car while I pop into the post office"? If not, why not?

The answer is yes, I would.

The reason? I don't think I would choose to say either (or even either note minor pronunciation difference). It would just come out of my mouth, in the same way that most everyday language does. I'd say that it was on a "whim" but it doesn't even reach that level of consciousness!