I know that this is my own hang up, but is there any reasonable way to adddress this without coming across as a crazy lady and making some poor service employee's day darker?
Honestly, I don't think there is a way. Speech habits, especially small unconscious ones, are difficult to change, and it's not like they're calling you something everyone recognizes as rude or insulting. I'm not trying to sound mean, but this is one of those things that I think lies within you, not in trying to correct other people who aren't wrong in the first place.
I agree. I don't see it as rude, even if it's not your preferred term. As long as the salesperson is being polite (reasonably helpful, polite tone, etc.) it would come off as nitpicky and frankly odd to tell them not to call you Miss, especially if you're just having a basic sales transaction.
Nobody uses Miss around here, and Ma'am is rare. Madame (I live in a largely bilingual city, French and English) is very common for all females over the age of about 15, however, regardless of marital status, presumed or otherwise . I've had a few people call me Mademoiselle, even though the word generally refers to young girls rather than any unmarried woman, which I thought was a little odd, but didn't seem to be said with any kind of ill intent. Mostly I've gotten Madame since I was about 14. None of the variations bother me, as long as the interaction is polite.
I think you just need to realize that this is a quirky pet peeve of yours (we've all got 'em) and just ignore it.