L.H.
Well, like Dr. Phil says, she's doing what's working for her. You can train her - you just have to be determined to win this battle - and you can! Be prepared to do what is necessary to stop it. If it means leaving the store and going home for time-out, do it until it works. And the other moms are right, too - you have lots of good advice here.
Oh, and don't feel guilty about being at wit's end - you haven't even started yet - my daughter is 15, and I can tell you, there are days that a fit in the store would be welcome!
I am exaggerating a bit, but never, ever give in on the the point you want to win. It's WAY easier to do it when they're little!
Hang in there, you'll do just fine!