asd and the grocery store

If you have a kiddo with asd, how do you keep them comfortable at grocery stores or anywhere that is busy? It seems like every time we go out it is a little bit worse, or maybe he is just getting bigger and harder to manage, I am not sure. Today he had a mega meltdown at walmart and almost broke his own pinky as i tried to carry him out... once it starts there's no turning back, there is no distracting him or calming him down, we have to leave and it can start over anything. Today's starter was 'Joshy please dont sniff the plungers' -.- so really, how do you do it? i know not everything works for every child but i am open to new ideas...
i usually have him hold the list and cross out items as we go, which seems to help sometimes, but today it was a last minute thing and we didnt have one

What works for my nephew is coloring books. Everywhere we go we keep crayons, paper, coloring books. He gets so into coloring it helps distract. Sometimes we task him with a mission like... We need banamas.. Can you draw a banana? If he has a good trip he gets a sticker for his board at home.

I let my son take a toy or his blanket. He's 4, and his daddy pitches a fit that he still carries it but I know how uncomfortable he can be in large crowds. Sometimes he goes without it, but he might pick up a toy in the store to play with or something. See, in your instance, I probably would have just let himm sniff the plunger. It's weird, and you would have gotten some looks, but it would have avoided the meltdown and it seems like a small battle to pick. But that's just me.