Your son is almost three, right? (Or am I confusing you with another poster who's planning a birthday party for their kid?)
When my daughter was about that age, as we walked into the store I'd remind her of the rules (stay in the cart or stay next to Mommy, use indoor voice, no begging, that sort of thing). We stopped offering so many bribes, but instead when I was in line on the way out, or getting back into the car, I'd praise my daughter for everything she did well in the store that day.
We also started to let her be a helper, which kept her engaged and entertained. We'd let her count eggs in the egg carton, or hold onto something while she rode in the cart. (She loved to hold onto bagged salads because she could hug them and they were so puffy and pillow-like.)
Also, I don't think "watch out" is that helpful, especially at that age when a lot of kids can be really literal. I think it's better to say, "Son, stop!" It's a clearer and more direct message for your son. Also, you won't run the risk of other people thinking that you're talking to them.