Well, for one thing only the state or local government may take out a tax, while a fee may be charged by anyone providing any service. A fee is usually connected to a special service while a tax is not.
Your bank may charge you a fee if you lose your card and need it replaced. It may also change a fee is you want to use the card in an ATM (which may seem absurd but that is what my bank does with my credit card)
On the other hand, the income tax that I pay to the municipal government I have to pay even if I don't use any of the services it provides. It is the same % of my salary whether I go to the (free) hospital ten times one month, or 0 times. It is not connected to any specific service.
Right now I can't think of an example where "taxa" should be translated into "tax". Even if there is 1 such example I think it's better to leave "tax" as a wrong answer, since "taxa" and "tax" are false friends. They seem to mean same thing, but they don't. It's good that Duo points that out to everyone.