When you book the rental car you pay only what is book it, or more fee be added.

Despite ours being in perfect condition there was a really hefty mileage charge. You need to clarify that with them although finding someone who knew what they were talking about was something of a challenge.

GPS is added, additional insurance, gasoline, you define what you want

Yes . It depends on the conditions you return the car. Mileage , gas and taxes .