I make a point of filling up to exactly "full" iand literally not one penny over. I keep the ignition turned on so I can see the lever change as I pump and I stop every dollar or so and wait for the lever to catch up before I continue. I also document everything and if they end up charging me and I have to call them and get it reversed, fine. But I'm never letting it go.

If Avis needs to pinch those pennies, so should I. They aren't getting one extra ML of gas out of me.