We give our son a countdown or set time to transition. I will tell him he has 5 more minutes and then we have to leave. I will repeatedly countdown 4, 3, 2 and then 1 minute left ( it doesn't have to be exactly timed). Start transitioning her with all her activities so as to avoid an unexpected tantrum. It is a useful tool but of course nothing is foolproof!

I completely understand your frustration with this and I'm so sorry! I agree with the visual schedule even if your child does have verbal communication skills many people on the autism spectrum are visual thinkers and this may be a huge helper! However, I am not going to lie a visual schedule is very time consuming for whoever makes it and it takes a lot to plan ahead in order for it to be successful. I just started a visual schedule and picture exchange system with my son and he is already doing wonderful with it! Another thing to consider is your daughter's sensory over-stimulation. If she is wanting to walk into the store and refuses the cart she might be seeking a certain sensory input to compensate the other sensory that might be driving her nuts! Even if she does talk (not sure if she does or not sorry) she might not be able to communicate how she is feeling exactly relating to the sensory issues. Does she have any auditory or visual defensiveness?

I have exactly the same problem . I always bring with me the gummy candies and I show him when we need to leave the store.
And I m still can carry him I don't know what I have to do when my son grow up..

Is there something that your daughter loves and always helps to calm her down? My son loves music, so whenever he has a tantrum, I pick him up and sing to him. It distracts him and calms him down.

We've done all of the above, with the addition of some distraction to move them forward and out of their little tantrum eddy. They just can't seem to move on by themselves. I used to do the countdown method or visual timer, then tell her she could have a reward/lollipop, whatever at the car. Also, I auto start the car so a DVD is running to distract from buckling in - that was a major physical fight in our case. Once the transition has been made, everything is ok. Kind of a trail of bread crumbs of sorts leading to the next transition. In your situation it may have been too late for any of that. Sorry it happened. It's quite a stressful event