I got a reply from Avis on FB:
thank you for letting us know about this. We are aware of this issue and are taking measures to correct it. Can you please send me your rental details and the description above to
[email protected]? Please also include "Facebook" in the subject of your message. Thank you!"
At least they claim that they know that it's their problem - another support for my charge back if I have to. I'd love to sort it out with Avis, even though I have filled in all the charge back forms and have all the documents ready.