Answer:
Ask her again to present the document.
Step-by-step explanation:
I would explain politely to her hat the law is very specific about not selling alcholic beverages to underage customers. In order to avoid any incoveniences for me and for her, I would insist that she present her document to confirm she is not underage. She should not be offended, and should be willing to cooperate by going to the car to get her ID or to leave.