Hertz family discount doesn't work anymore. The Costco codes do not waive the underage fee. Your best bet is to join CAA and use the AAA or CAA code with Hertz. They can generally be used interchangeably for the most part as there's reciprocity between AAA and CAA for discounts in most cases. If you specify your age at Hertz.com as under 25 and it doesn't add the underage fee on the checkout page, you should be good-to-go.