Looks like Avis and Budget may be changing policies and multiple reservations will not be allowed (try using a code you are not eligible and then using a backup) without canceling ahead of time.

One potential benefit, they might be better able to keep the car reserved in stock and not have to give replacement vehicles or run-out during high demand, but I doubt it.

"Global distribution system operators are preparing by December to take credit card information for bookings with Avis Budget Group in the event the car rental firm decides to apply a new policy for no-show fees."
and
"So, while for some locations and time periods we may choose to impose a no-show fee, for others we may choose not to. Our new capability will align the car rental industry with similar approaches in use by airlines, hotels and cruise lines. It is a long-overdue change."

management.travel article

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