What? By pushing back against a customer who wishes to be addressed as Mrs, you are invalidating their own sense of identity under the guise of doing them a favor. It's paternalistic and infantilizing.
EDIT: Oh wait, "customer" here may mean "client", as wilg pointed out, ie. the person asking for the form to be built, not the person filling it out. That is likely the source of this misunderstanding. I still don't see the harm in providing both Ms. and Mrs./Miss -- some surely prefer Mrs. and there is no shame in deciding you prefer "Ms."
EDIT: Oh wait, "customer" here may mean "client", as wilg pointed out, ie. the person asking for the form to be built, not the person filling it out. That is likely the source of this misunderstanding. I still don't see the harm in providing both Ms. and Mrs./Miss -- some surely prefer Mrs. and there is no shame in deciding you prefer "Ms."