I would strongly suggest not leaving her in the car at all without supervision. It only takes a few minutes for the car to heat up enough to kill the dog. Is there any way to take turns watching the dog while others shop/eat/etc? Can you leave the car running with the airconditioner on (another set of keys perhaps)?

There are too many horror stories about dogs that die when left in a car for even a short period of time. (Babies too)

I don't know where you are located - maybe in your area it is not hot enough yet to be concerned, but here in New York it is 80o outside which would make the car about 110o in a few minutes.

Good Luck