I just recently bought a car last week and when I put a down payment on my Amex, the sales person said they only take up to 5k with credit card else they significantly lower his commission. I was fine with that since I want planning on putting 2.5k down anyways. Then after filling out the paper work and handed loan officer the card, she basically encouraged me to pay with a different card. I asked her if people pay more with Amex than other cards and she replied that they once cancelled taking amex because of the fees but when they noticed lots of potential customers leaving because they couldn't use Amex to pay, they allowed Amex. I would have used Chase but I wanted to give a little something to Amex since all I use them is for 6% groceries.
Have you tried calling Amex and asking them to allow that dealership to purchase the full price vehicle to a Centurion member?