One thing I have noticed about costco rentals is that when you book in the 24 hour increments, the rate is cheap than if you book in say 20 hour increments.

For example I was looking at a car in DFW I picked 5pm pick up time on 11/18 with a return of 3pm on 11/21. The costco rate for budget was $120 for SUV at Budget. However, when I changed to 5pm on 11/21 then the rate dropped to $102 same SUV same Budget.

Similarly, looking at a car in YEG Dec 25 6pm to Dec 31 4pm, = Alamo $252 mid size. When I change the time to 6pm on 12/31 rate drops to $234 same Alamo same mid size.

As rental car use the 24 hr clock, even if you return early, they will not change price unless it is more than 12 hours early or something like that.

This has happened to me at least three times now so I always book in 24 hour increments now.