We also use COSTCO for booking rental cars. I continue to recheck the rate frequently before date of departure and almost always find that it has dropped so am able to book again at the lower rate and cancel the original. Lately we have been opting for Budget b/c as a Fastbreak customer, there is no wait and Budget periodically sends us $25 off coupons.
XVP -- rental cars
Does Costco accept coupons at the time the reservation is made, or do you have to do something with the car rental company?
You actually make the reservation with the rental company through a portal from the Costco website. Any selected discount codes are automatically entered into your reservation.
Alternatively, you can do what I do and make a note of the desired discount codes and then manually log into my account at the rental website, preserving any additional benefits I have there under my avvound (e.g. Avis Preferred/First, Budget Fastbreak, etc). I then manually enter the codes into a reservation request.