Successfully used the Visa Cardholders Discount Code (B204400) with Avis for two different rentals in Australia. Booked on avis.com.

First rental pick-up - zero problems, fee waived
Second rental pick-up - guy tried to charge the underage fee, but gladly removed it once we mentioned that the code negated this fee from our online booking

Saved hundreds of dollars because of this. Thanks to whoever found it!