REQUEST: Car rental for a week (Dec 24th to Dec31)

Hello all,
I am trying to rent a car to drive from San jose to Los Angeles and back from Dec 24th to Dec 31st. Would like a standard SUV or minivan(4 adults and 1 kid) and the best rate i have with unlimited mileage is $540(without discounts)

I get these discounts as i am a member of Costco
Avis: AWD: A108326 (25% off with this)
Coupon : MUGA013 ($50 off)

Now Avis says that they will calculate the discounts only at the rental counter and with 25% off and 50$ off because of the coupon it comes to $355 which i think is sweet for a standard SUV for a week.

Now my question : Is there a way to confirm that the Costco coupon will not be denied at all or do i have to only get it at the rental counter. The expiration date for this offer is April 2010.

http://www.costcotravel.com/#15_rentalCars_avis_AVSGTSMUGA013200...

Thanks for the help..

VJ