I went to costco.com, clicked on the travel link and it brought up all sorts of car rental companies. Then when I went to each one they already had the Costco discounted coded.
I got a good deal on an SUV rental, under $300 for 8 days. It was the best I could find out there right now.
We just used expedia.ca...Dollar had the best deals...we had a Sebring (plenty large for family of four) for 8 days for under $200CDN (including taxes)...counter pickup and dropoff right at the airport...it was a good deal