I used a harness on my 3 year old son to prevent him from running into the street when we went shopping. People looked at me like I was not a competent parent. My son had twice before ran into the street in traffic before I could load him and groceries into the car.
Yeah, gotta agree with Johnny. Put the kid in the cart. Take the kid out of the cart and place said kid into the car/child seat. Put groceries into the car. Put cart away. It is not difficult, and I speak from experience - I have a nearly 5 year old son who would dart given half the chance, especially when he was three. There are other ways to control your child that you’ve chosen not to use.