I'm not clear if its been said already, but the first rule is the customer is always right. Yes there are exceptions, but in this case if all they're doing is mis-gendering you either intentionally or unintentionally there's really nothing you can do about. Get bitchy and they complain. Your employer can terminate you and its not trans discrimination becuase your behavior is causing them to lose customers (money). They also have a pretty strong foundation to deny you unemployment then as well.
If its really uncomfortable for you, find a new job, just don't paint yourself as someone who's going to be an HR problem.
