I have never been a cashier, and am not a lawyer or a authority on Jewish law. It seems to me that the employer is suppose to pay his workers for the time spent on the job. Once the supervisor turns the light off it seems to me that the cashier is to close the register and bring the cash receipts to the office so that she can clock out on time. Ask the supervisor as soon as he turns off the light if you are going to be paid if you stay beyond your regular hours.
It should be noted that if you do ask to be paid for your time, you maybe fired r”l. I think you should talk to a Rav and a labor attorney before confronting your supervisor. Are cashier jobs readily available in your area for the same wages? Call a couple of stores on your break assuming you have a cell phone and ask if they need cashiers you will be surprised how easy it is. This way you can have a new job lined up if you get fired r”l or a second job if you need the income.