WOW! Talking with station agents before, they've said that the only way they can charge for smoking in the car is with proof in the car (cigarette butts, ashes, etc) that they can capture with pictures.
If they do charge your card, I'd:
1. Contact Avis on Twitter
2. Contact the Better Business Bureau (I've found this is the best way to get a reply from Avis)
3. Dispute on your credit card