This just recently happened to me except I was charged $840. I returned the vehicle at Miami International Airport to AVIS at 4:30am before opening hours. I was told at the airport it could be returned 24 hours a day by just returning the keys in the lockbox. I never got the receipt showing closed out. Today I see the charge on my card and look up my confirmation number on their website and it shows I returned the vehicle 8 days later at a completely different airport (Orlando). I've disputed and they said it could take up to 3-5 business day. Certainly they have tracking devices to show the vehicle was indeed dropped off. My flight left Miami the day I returned at 6am.