The last time we rented from Hertz (and by 'last time', I don't just mean just 'most recent') they attempted to invoke a bogus 'smoking in the car' penalty, after having taken various steps to hide the existence of a residual smoke smell before we drove off the lot. Fortunately, we were still in the area when the bogus charge appeared on the email receipt, and after we confronted the perpetrator's co-workers in person (the perpetrator himself conveniently taking a break at the time - perhaps smoking in one of their other cars), the charge was dropped.
Costcotravel.com is the way we do it now, too.