Do not buy gas from the rental company. Never!
Most of the rent a car company is requiring from you to return the car with a full tank. If you don’t do it, then you will be charged for the gas you didn’t put in the car. Of course, I must highlight that you will be charged at rental car rates. So advice to you is that on your way back to return your rental car, you simply stop at the local gas station and fill the tank.
You can avoid the extra airport fee and save extra charges.
Let’s say that you have traveled by plane and just arrived at the airport try to avoid to rent a car from the local counter at the airport terminal as you will get a surcharge for an airport fee. It is unfortunately legally obligated from rent a car company to collect and pay these surcharges to the airport, so you are unable to avoid it if you rent a car at the airport.
You can save a lot of money by bringing your own extras.
You should always pack your own traveling extras and ensure that rental company has removed theirs - from the car and the bill - before you drive off the lot.
The best tip I can give is that you research and ask all the questions that come to your mind so you could be aware what exactly you are getting and what are you paying for.