You can also cut out the middle man and confusion of different reservation numbers by reserving directly through Alamo and put in the Costco Contract ID yourself on the reservation along with any valid cash, % off or free upgrade coupon code (can be used together, Google coupon codes). There is a place for it near the bottom where is says "add coupon" that you can enter Corp and coupon numbers. You will get the same rates and the same free additional driver. Always sign up for Alamo Insider and add your info ahead of time, as an insider and info on file you will always automatically have accelerated check in. You can skip lines with the kiosk.
If you have an American Express card, you can get full coverage insurance for one single cost of 24.95 per rental period. It is charged on your credit card only when you rent a car and pay with your American Express card. It is the same coverage you would pay 24.95+ A Day through Car Rental company Its great!. Call your AE card to find out the details. It is in place of your own car insurance for peace of mind to walk away if the car was totaled. I think reg credit cards cover a certain amount after insurance. Anyway something to look into if you are interested.