It's honorable to have admitted to "slow reading" LOL...So I'll have to repeat:
1. Costco codes can be used when booking "directly" on rental co's sites. There is zero advantage to enter anyone's booking on costcotravel.com who is a third party in car rental transaction between car rental companies and consumer. No such thing as booking "directly" thru third party.

A few months ago, costcotravel.com did have a $20 Costco giftcard as an incentive to book thru their site. Should they run it again, I may

2. Major pitfall of costcotravel is "partnering" with merely 4 rental co's, none of whom delivers consistently great cars/customer service.

3. Costcotravel futher undercuts consumer's advantage by NOT partnering with a single cashback site

So overall: while costco codes may prove to give consumer best rate (in minority of cases; not "always" by an extreeeemely long shot), a number of drawbacks are guaranteed:
1. If you do book on costcotravel.com, you're not booking directly
2. You can't use CB sites and easy referral bonuses currently $10-25 each
3. You get your cars from a limited number of inferior rental companies

So besides the giftcard scenario above - currently unavailable - I'm still waiting for someone to come up with a scenario under which a consumer should value car rental opportunities presented by costcotravel.com; that being said, I do routinely look them up for research. What are other great places for research?