Hi Angelique, you know, maybe you should've confront her right there rather than just look nice and smile? Not sure if you are in a position to stand up for yourself at work when servicing your customers, but if you are, making a short statement that you expect courtesy of being called with the gender you're associating if they expect any courtesy from you as your customers.
And if she's not understanding, I'd start calling her honey and flirting with her husband

or ignoring her - anything to show them their business isn't much appreciated
Obviously, it's all good if you are in a strong position on your job and your manager / store owner would stand for you. Otherwise, .... it's good to imagine in your head (or call for less drastic measures would be needed, maybe just stop smiling and show your disappointment with them without actually calling for a confrontation).
And do you wear your name tag? Sometimes a little thing, such as female name on your chest would cast enough doubt in customer's mind to avoid using any gendered pronouns).