I hope that you fought them on this and they reversed the charges. There is no way that you should have to pay the gas. It might say so in the contract but even the most basic customer service would dictate that you should get the gas for free and even a discount on that rental or coupons for a future rental. It might not be Avis's fault but even I, who is vocal at times, would hope that Avis would treat you better than this.

Please let us know how it turned out.