Are you sure they are calling you "Miss" and not using the generic "Ms." I'm a cashier in a convenience store, and my interactions with customers are usually not prolonged enough for me to even concern myself with marital status, or looking for clues. It's "Ms" if you look under 40 or so, "ma'am" if you look older.
To be honest, that's what bugs me. I strongly dislike women having different titles based on age. A 20 year old man and an 80 year old man are both sir so I don't see why women can't have the same title no matter what their age. I think many women take offense to ma'am because they feel the cashier is implying they look old. I think ma'am is technically proper for all ages.
And I think that will happen eventually, it just takes a while for those old connotations to fade away in peoples minds.
I'm in Australia, and was a teacher for 9 years before kids. In my experience, it's uncommon for female teachers to use Ms, rather than Miss or Mrs. That might be different in the business world, I don't know. I suspect that eventually, Ms will become a lot more prevalent, but it will take a while. Then we can avoid the situation I had a few years ago where the four year 7 teachers were Mrs McAuley, Miss McEvan, Miss Morris and Ms Minner. Very confusing for kids and staff alike!