I always get a receipt indicating that the gas was full on return. The only company who I've received a post-return fuel fillup charge on my credit card was with Hertz. But luckily I kept my receipt from Hertz which showed the gas tank was full.

I disputed it w/ my CC company and sent them the receipt. They wanted to charge me $78 for a missing 8 gallons. LOL. How in the world can I be missing 8 gallons when the fuel tank showed full upon return?