If you want to avoid working extra hours for free, the best option you have is to talk to your co-workers and find out how they feel about it. If everyone agrees to leave the store as soon as they clock out, then you've got the strength in numbers necessary to make a change. In your question you said that your boss told you explicitly that he or she doesn't want anyone working off the clock, so it's unlikely that they will explicitly press you to do so. And once your employers realizes that they can't expect you to do extra work for free, they'll have to hire more workers or pay you for your time. But remember, if the boss can get any of you to continue to do this extra work, he or she will use it against all of you. This one is really in your hands.