Problem building a smart shopping cart
I have a couple of ideas for you but know these are only suggestions. No hate please.

Have you tought of IR following?

IR is a almost unique signal that only you have. You can either use multiple IR sensors to determine where the emitter is or you can use the servo motor to do it as a radar. Note that you have to isolate each sensor so it knows where you are and it doesn't captures the wave from a different angle by accident.

Extra safety?

You can also incorporate an untrasonic distance sensor so that the cart doesn't bump into you or any one. Like you can say, if the distance is smaller then 12 inchs / 30 cm, stop the cart. It is practically an extra safety feature.