Having 13 years’ work experience on the Ukrainian market we as anyone else understand value of your time and aim to save it even in smallest details.
You get a car with full fuel tank. After the end of rental term you also need to fill it with fuel.
You did not have time to refuel car on its return to the Avis station? This is not a problem. In this case you need to pay for the rest of the fuel on tariff $1,98 for 1 liter (including VAT).