Always ask a manager if they want you to stay later. Don't just stay later because you feel obligated to on your own. If they don't need you to stay later then you clock out and leave. That's normal in retail, if they're busy or they don't have any one to cover a shift they can ask you to stay later. Make sure you're being payed for the extra hours. Overtime doesn't always exist in retail specially if you're part time, but you would have to check labor laws or the companies policy with regards to overtime. If you're still unsure, check your next pay check (any overtime should be listed on it) or ask about overtime pay if you're constantly staying later then you're supposed to.
Been there, done that. Make sure that you get paid for the time*, for the extra hours. In the US it is against Federal Labour Laws to NOT get paid for hours worked, and Not get *Overtime pay after 40 hours in a single week.
If you are not getting paid, you also don't qualify if injured during that time for Workman's Compensation Insurance.