I understand that in terms of context - yes, this means 'What are you having? '.
However, it's actually not though in translation- it's 'what are you giving yourself?' which is of course nonsense in English.
However it's not uncommon for a bartender or server to ask 'what will it be?' in English.
If you don't want to allow this - I think it's a bit unfair to have the choices available to build this (what will it be) sentence. As the answer is never going to be a direct translation and always a familiar phrase.