I'd advise you to use "ma'am" unless they look like they're under the age of 18.
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AnonymousI'd advise not using 'ma'am' at all or ever, as statistically 4/5 women are deeply offended by this sexist terminology that judges women based on their age. There is no 'young sir' 'old sir' terminology in place so avoid 'miss' and 'ma'am' differentiations.Simply do not address people at all, just be polite about your business with them.