I really wish there were such a word. I use "sir" and "ma'am" a lot professionally, though I tend to use it more often with patrons who are older or seem cranky. People my own age I'm less likely to use it with. But being aware of the fact that there are people who are genderqueer (or who aren't, but whose names or vocal ranges make most people assume the wrong gender, always fun on the phone) really does make me wish there were a word that meant "person I am currently providing service to".

(It also always amuses me when they call me "ma'am", because it seems to me a complete misread of the power dynamics of the situation, but those people are usually southern)