Renters pay for the fuel by returning the car with the same amount of fuel as you gave to them at the beginning of the booking. Agreements concerning fuel and payment are settled amongst renter and car owner.
We advise that you, as owner, provide a full tank and the renter returns the car with a full tank as well. This is the easiest way to handle this.
If the car comes back with a different amount of fuel than it left, you find a solution together. Make sure you have an agreement with the renter on this topic before handing over the keys.