She obviously shouldn't have behaved the way she did in the supermarket but the way you describe it sounds as if she was a bit desperate - either for a bit more control over her own life - or for some other reason.
Is this sort of behaviour out of character for her?
Do you often do things just the two of you together?
Is there anything going on (other than hormones) that is worrying her ie will she be starting secondary school in Sept?
Why was she so desperate for the baking materials? Was it something to do with her friends?
Sorry for Spanish inquisition! Have total sympathy as I have a 12 yr old who has started to fly off the handle for the most ridiculous reasons. And I sometimes feel the more treats she has, or the more people try and accommodate her, the worse she gets! It is really difficult.
Are you able to go in and talk to her? Tell her you were very disappointed (and bewildered) by her behaviour at the supermarket, especially as you had been reasonable and let her have her own way this morning.
Ask her what made her behave that way?
If she has calmed down sufficiently, could you ask her if she felt she had behaved in a reasonable manner? Ask her what she thinks a reasonable punishment would be?
And then try and focus on future solutions ie she can choose what she buys in the supermarket up to a certain amount if she pays out of her own money and sticks to a budget or does some chores in exchange or some such. But then make it clear you won't tolerate such behaviour again and if there is a repeat performance (esp with regard to pushing etc) you will be leaving shop immediately and you won't be giving her lifts anywhere for x number of days.
Good luck with it all.