The years during which a child is becoming aware of herself as a separate being, with desires and needs of her own, are particularly challenging, because she has parents who want and need an entirely different outcome of events on their own predetermined schedules. Go to Payless? What a wonderland of things to touch, examine, play with! Shelves and shelves full of curiosities!
Will mom let that happen? Nope. No time, too much risk of breakage, maybe other kids to keep track of, worry about other adults' eardrums and judgements. So the child is told NO! to almost everything she wants. Is the child frustrated? Well, um, yes. And going to the store becomes something entirely too frustrating for the average 1-to-3 year old.
So, add the complication of this happening to the child more or less continuously in all sorts of situations, the child being short, at her age, of impulse control or emotional alternatives, having poor language skills to express her longings, having virtually no power to change anything, and possibly being tired, hungry, or already over-manipulated. The ages of around 2 through 3 are extremely difficult and frustrating for kids, and also for parents who don't understand why they are so frustrated.
I've found it much more helpful and less stressful, rather than to correct or punish that inevitable frustration, to anticipate and allow for it if we can't avoid it. You are learning when your daughter is most likely to have a meltdown. To whatever degree possible, provide calm and cheerful distraction and redirection. Be sure she's fed and rested before trips out. Be aware that all those consumer goods are attractive to her, and give her supervised chances to pick up and examine things. Talk to her about what they're for, what they do. Find ways to say "yes" to her as often as possible, by anticipating her legitimate interests and needs. Give as many positive choices as possible – this requires some thoughtful planning, but it's so worth it. For example, "Should we go down this aisle, or that aisle? Do you want to look at the sandals first, or the tennies?" And keep a "special" toy for shopping trips – something that she gets to play with at no other time, to hand her when she's getting bored or frustrated with all she can't have.
You mention that you know why she's getting upset, so work with what you already know. Your daughter desperately needs this from you, because she's clearly expressing needs that are not being met. Find ways to avoid the situation, or learn to empathize with her very strong desires. Often, just having an adult help put words to her wishes, and knowing you care, can completely shift the energy into something more positive and playful. Check out how Dr. Harvey Karp, author of The Happiest Toddler on the Block, accomplishes this miracle here: http://www.youtube.com/watch?v=fJ1428uYs2g&NR=1&f...