The cars must be returned with a full tank of gas. If not, the rental car agency will refill the tank and charge for the service. It is often cheaper to fill the tank at a local gas station before returning the car than to pay the rental company's per-liter rate.
Every driver must pay for their own fines. A Statutory Declaration that identifies you as the driver will be created and sent to the relevant authority. An administration fee will be charged to your credit card once the transaction is complete.