I did this on my February trip. NEVER AGAIN!!
I rented from Hertz since we needed a large SUV and they were the only comapny that had it available(their rates were very high and would never use them again). I managed to use some codes to get the rate down. When I went to book it over the phone Hertz's computer was not set up to use the AMEX point certificate.
Now I hadn't ordered my certificates from AMex yet as you shouldn't do that unless you are sure that they would be accepted. I was told by the person booking that I should go ahead and order them and to call a week before my trip and see if it accepts them. Being the neurotic
planner that I am I tried to explain to the person that I need to have this finalized today. She assured me that there would be no problem using the certificates they have a system wide problem at the time that was not allowing to put the certifcate codes in. If at the time I called they were still not able to do that. I will not have a problem just giving them to the counter rep at MCO. Oh and ....I was getting a triple A discount as well...the woman told me that I could combine the codes.
This should have been my first clue that I was getting wrong information. To make a long story short. I was quoted three different prices I ordered my AMex certificates and they were accepted.
Here is the kicker when I went to MCO counter they looked at me like I had 3 heads when I presented the certificates and then they proceeded to tell me that the reservatiopn should not have been booked that way with the
AAA discount and AMEX certificates. ...you know they are probably right but that is what they quoted me. She said she would have to rebook and it came out $300 more....imagine my surprise
. Being the neurotic planner that I am I had every persons name, date and time I spoke to including their id#'s. SO that covered me but I wasted a ton of time at that counter. Needles to say it was still $50 more than the original price quoted but I was tired of arguing
the kids were getting unruley and I quite frankly wanted to see the mouse
. So I gave in and knew I could resolve it by mail.
When we returned the car my brother check us in and I noticed the slip was $150 more than what I was quoted at the counter. I was fuming.
..and they immeadiately corrected it.
So in short I would never use the certificates for rental cars and yes I did complain to Hertz customer service and received $100 certificate (can be used like cash No restrictions) from Hertz Customer service. Which I will be using for my South Florida trip in July.
My mistake I didn't see the word "travlers check". I guess I am still traumatized by Hertz from my February trip. I would think the travlers checks would be fine ...except that you would have to go inside and hand them in at the time of rental. I like to just hop in my vehicle and off we go. 
