D.K.
He doesn't understand explanations (regardless of what other posters say). Things that will help are
1. setting him up for success - make sure he is not hungry or tired before setting out on a trip/going somewhere likely to be a problem. Does he get sleepy/grumpy after lunch? Then that's probably not the best time to go shopping. Avoiding the triggers for tantrums is way easier than managing them once they start.
2. Say yes a lot. And tell/show him what he can do. You don't want him to grab things from the cart? Give him something fun he can grab. Make it fun - Costco often has samples to try - try the papaya or pineapple or whole grain bread with him.
3. He does need to run around. Do some running around before you go to the store.
4. You can hit him. He will learn that big people hit little people. I can't see how that will make going out less frustrating to him - and frustration about being powerless and unable to communicate is what tantrums are all about.