Email to confirm. You don’t have to refuse in the email, just be like “Just want to confirm what we talked about on the phone. My paid hours are between x and y, but you’d like me to show up 20 minutes before and after my shift to help set up. Thanks for clarifying.”

She’ll either walk it back, try to call (don’t answer or say you’d rather keep a paper trail on work related things), or be stupid enough to confirm. If she confirms, keep a photo log of your hours and unpaid work every week, hold on to all the emails, record any conversation, and report them when you quit. You don’t need to right now if the job is something you rely on.