Looks like AMEX Platinum has added Avis to its list:

American Express is pleased to welcome Avis to our list of partners. As a Platinum Card member, you’ll enjoy complimentary membership in their Preferred Service program, which means you can count on special privileges and an easier rental experience with faster reservations, no paperwork and no long lines. You will also receive the lowest mileage cars and best parking spots.

As a Platinum Card member, you are also entitled to the following special discounts1 on the normal time and mileage rates at participating Avis locations:
15% off local daily and weekly rates in Canada and the U.S.
5% off local weekend rates in Canada and the U.S.
10% off Intercity rates in Canada; 5% off Intercity rates in the U.S.
Up to 20% off rates outside of Canada and the U.S*
5% off promotional rates in Canada and the U.S.
Just Charge your rental to your Platinum Card and quote your Avis Worldwide Discount (AWD) #CO76700 to enjoy your special savings.

For more information, visit http://www.avis.com/AvisWeb/html/bri...o.html?C076700

* Applicable amount vary for daily, weekend and weekly rental in Europe, Spain, Japan, Africa, Middle East, Latin America and Caribbean

1 Discounts apply to the time and kilometer/mileage charges only and not to any other charges in connection with the rental. Blackout periods may apply. Rates may vary by location. Certain restrictions may apply. The terms of Avis' standard rental agreement apply. For Full terms and conditions please visit avis.ca