I'm sure you know your child best and as long as you've ruled out any medical or physical reasons as to why this is happening, then I'd have to agree he's just being defiant and testing his limits with you, and - unfortunately, sounds like he's winning. In order to not spend the next 10 years and beyond dealing with this behavior, you have to nip it in the bud right away.
Two rules with kids I learned at a very young age, 1) don't start any habit you will need to correct later and 2) consistency, consistency, consistency.
For instance, bribing a child with a soda and a popcorn in order to sit nicely in the cart so you can shop - - no way. (My sister did this with her daughter and it drove me nuts to watch!!) Not your situation, but it has always been my number one rule anyway!
Secondly, pacifying a tantrum-throwing child with anything is simply unacceptable. All you've done with that is taught the child that the unacceptable behavior will be rewared with whatever "treat" they've managed to get from you. You have to hold your ground and not give in. Once you give in, the child "has you". They know given the right tantrum, at the right time and place and you'll melt like butter in their hand and do whatever it takes to resolve the situation. Once you start standing your ground and being consistent, it will stop. BUT- I promise it will get worst before it gets better because in their mind, they're saying "it worked yesterday, why isn't it working today? I guess I better scream a bit louder, a bit longer"... and on about day three when you've had enough, you cannot give in to it. It will stop on day four if you can keep it together one more day. Otherwise, you've lost 3 solid days of progress; even though it doesn't appear to be progress, you'll be back to square one. They get the new toy they're screaming about. Now you've taught them persistence. They are thinking "Yep- she really doesn't mean business at all - I just have to be a bit louder, a bit longer and then BINGO - I get what I want."
You are best to set up some practice sessions and plan on atleast a week of hard times. One week - such a short amount of time in the big scheme of things really. When the behavior starts, in a totally calm and emotionless tone, tell him "That behavior is unexceptable and if you don't stop it, we are going home." The absolute instance it happens again, the child is removed from the cart, the half full buggy of groceries is promptly deposited at the Customer Service counter and you immediately drive straight home, without addressing the child at all, no words, no eye contact, nothing. Once at home, you calmly tell your son "This is what will happen from now on if you cannot listen and behave in an unexceptable manner." Then there are no trips out for the rest of the day (thankfully Meijer is open 24/7 and you can run back once the kids are in bed and asleep with dad or a babysitter and you can pick up whatever you really needed.)
Your child will know you mean business because you are managing the situation unlike anything he has seen before. He will be stunned. He didn't elicit an emotional response from you, there were no idol threats - you explained what would happen to him in clear, simple terms, and then it happened and he didn't have a chance to escalate the situation to another level and "win".
I know it sounds easy on paper, and like I mentioned before, it gets worse before it gets better. But I promise it works - I have two 11 year olds as proof.
Once you're over the "hump", be sure to reward the good behavior. Kids need praise, love and attention by the buckets full. You have to make it so the desired response from you comes from doing things "right". You have to notice every little detail and make sure it's noted as important and appreciated. "Wow - you did a fantastic job of picking up your toys!" "Hey, I had fun helping you with that." "You know, you're the best kid ever!" You need to find 20 ways a day to compliment and build your son up in a positive light so he sees the rewards of his good behavior. I know I enjoy my job a little more when my boss tells me I did something right- certainly you son will as well.
Sorry for the long reply - I just got off on a tangent. I hope there was a piece or two of advice that will help you on your way!