Don't know if this code has been reported before, but this is from an ad on page 7 of this month's Costco magazine:

$25 off a weekly rental

Rent an Avis luxury car, convertible, sports car, SUV, minivan, passenger van, or specialty car for a minimum of 5 consecutive days and you can receive $25 off your rental.

Include your Avis Worldwide Discount (AWD) #A108300 and Coupon # MUGH010 in your reservation. Clip this coupon and present it at the Avis counter upon arrival. Please be prepared to show your Costco membership when picking up your rental car.

Rental must begin by 3/31/08.