Yes. No difference to book in the rental car website with Costco codes.As you mentioned, you can get cashback if booking direct. For me it is just more convenient to use one website to check price of several vendors.
Originally Posted by
SinglePapa2
OP leads toward more consequential discussion: why book at Costco Travel's website? (unless it's giving you an advantageous price error). Before payment, Costco site lists codes used. Take codes, and book at car rental co's site. Besides dealing direct (which has advantages), you may also be able to start the booking via a cash-back site