Translation:– One milkshake, please. – Here you are.
This has been partially answered in one of the other comments. 'You're welcome' cannot be used in this instance because when you are giving something to someone else (in this case the milkshake to the person who requested it) you would never say 'you're welcome'. 'Here you are' is the natural thing to say. As has been said in the other comments, you only say (unless you're bringing up your children...) 'you're welcome' if somebody has expressed gratitude - either by saying 'thank you' or 'I really appreciate your help' or similar.
Ole hyvä usually means you're welcome and is used when someone says kiitos first. But in this case when kiitos translates to please instead of thank you ole hyvä sounds wrong by itself and I think better thing to say is "tässä ole hyvä" or "kas tässä ole hyvä". I mean you wouldn't say you're welcome when handing someone something unless they say thank you first, right? :)