Cash back referral bonus are are different than cash back. Referral bonus is paid if you refer new users to cash back site.

Cash back from the transaction are usually in the range of 1-3% range or $3-$4 per booking. Take a $400 rental (and I'm being generous by assuming the percentage back includes the tax/fees etc). 3% is $12. This savings is small compared to not using the Costco discount in the scenarios where it is the cheapest and sometimes by a lot more than $12.

I get it that you can take the Costco code, and go to cash back site to book thru OTA or directly with rental agency but again this thread is about the Costco discount itself and not whether you can combine cash back and/or pay with a card giving extra bonus for car rentals for even more savings. It's great you point out that we can get extra savings this way but boycotting Costco's site!? This goes back to my earlier post, you can make the argument that buying directly on Best Buy, Starbucks, Kohls, Columbia Sportswear, Walmart, Macy's is not the cheapest unless you go thru a cash back site.

As to the referral bonus, the cash back sites have conditions against self referral. Seems like this would be skirting their rules to setup cash back dummy accounts each time you book a car rental. Boycotting costcotravel.com because it does not participate in cash back prevents one from obtaining a $25 dummy referral?!?!?