I tried this once, after confirming with AmEx that the charge would be approved (in case the dealer was actually willing to do it). They declined. I pointed out, politely, that their merchant agreement requires them to take it. The salesman went off and consulted with the sales manager. He then offered the LoJack, which I had ordered, free if I paid for it some other way. Since that was more of a discount than the miles were worth, but cost them a lot less than the credit card fee would have been, it was a win-win. So, you may be able to get something out of this even if it's not miles.