According to customer FAQ on Amex.com
I went to https://faq.amextravel.com/app/answers/list/c/2219,2306/kw/Membership%20Rewards%20Car
and clicked on the link for the question "can I use membership rewards for car rentals".. it states:
At this time, it is not possible to use Membership Rewards® points for car rental reservations. This is due to the fact that payment for most car rental bookings occurs upon return of the car and bookings are not pre-paid.
So this isn't saying you can't use the points for car rentals, just that you can't use them to reserve the car.
Then I clicked on the question "In the past, I have redeemed membership rewards points for car rentals... am I still able to do this today?
You still have the ability to redeem points with our travel partners by redeeming points for car rental certificates. For more information, please click here. (which takes you to a page where you have to log in to your amex account to see what is available for you to "purchase" with your points.
What you should do is see what certificates are available to you to redeem and then carefully read the terms and conditions of use and follow any instructions that are required to use the certificates. The car rental company that honours the certificates may have special requirements for their use.
The first step should be to contact customer service for amex to clarify what you can and can't do.
The phone number for Flight, Hotel, Car Online Support is 1-800-297-2977
supposed to get the certificate numbers before you book the reservation.
I went to www.avis.com/amexMR and it stated that you need to check rates and availability (by entering dates, times and rental location and clicking "continue" to see eligible vehicles and rates.) before redeeming your rewards.
Then you would complete the Membership Rewards redemption process to get your certificate(s). After you got your certificate numbers, then they want you to go back to make the reservation.
To see if you can try and pull up your reservation and add the certificates, try going to
At the bottom of the page there is a section that asks Already Have a Reservation?
where you enter your reservation number and last name - see if you can pull up your res and if it gives you those fields to enter your certificate number and serial numbers. (after the car selection and optional equipment pages).
Then I went to the Amex site to check to see their instructions to see if they had a way to add certificate numbers to an existing reservation. It also said not to make the reservation without the serial numbers of the certificates.
They also gave a number to call Avis 1-800-962-1434. They say that you must tell the Avis representative that you speak to, that you will be using Membership Reward certificates. Mention AWD# K004500, Coupon MUEA008 and provide your personal Avis Serial Number(s).
That phone number appears to be different from the normal res line and was not a number that was listed on Avis.com, so it's worth a try. If you can't get any help there, I would suggest calling Customer Service at 1-800-352-790
I think that if it were me, I would want confirmation from the customer service department rather than just bringing the certificates to the rental counter as you were told and hoping that the certificates could be applied. If you get the same response from Customer Service, I would request written confirmation that you could bring to the rental counter.
Hope this helps!