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!