Translation:Altogether, you will have to pay fifty euros.
It depends how picky they are. Yours sounds like an obligation and demand for money, and theirs could be something you might tell somebody who is going to buy something and will have to pay in the future.
If you are asking me what it will cost for you to rent a car at the airport, I might look at the price, look at taxes and insurance and tell you that altogether, you will have to pay 50 euros. Telling you that you must pay $50 euros would be wrong. It would be like leaving out the word "will" in the given answer.
On the other hand, if a cashier said that you will have to pay $50 euros, it could be understood to mean the same thing as what you responded with. But it would be a strange way of wording it, except in certain conditions.
I'd say that the overlap in meanings is narrow enough that I wouldn't argue the point.