The OP stated he had a gas receipt but that it didn't make any difference. It was clearly an error. I was only asked for a gas receipt once (by Thrify at BOS) and was warned in advance that it would be required. Gas gauges can be a problem so I suppose having the receipt can be used in your favor in a dispute. But the receipt doesn't state which car received the fuel anyway. The extra charges by Avis at LAS are not in place at other sites in my experience. All charges are presented when you accept the rental - get it clear before you take the car out. If there is an airport fee, point out that it is included in your Priceline contract.
It is not an error. It has happened to me on several occasions, which is one reason why I avoid AVIS now. I sincerely believe they are making money off the people who don't check their receipts or don't take the time to complain.