They cannot "simply refuse the card if its American Express" unless they don't take Amex. They must accept Amex under the same terms and conditions that they would accept any other card that they accept. The difference is that VI and MC have the SIC of New or Used car sales on the Restricted merchant type list. So they set up those accounts for service departments. Amex will set up a merch under the sic code of new or used car sales. The response was in regards to the statement that they "must accept your card or report them". That is correct they must accept the card but they do not have to accept it for new or used car sales. They can agree to accept it and tell you that the "cc price is 3% more than the cash discount price quoted" you can either pay the difference or not do it.

I routinely purchase my cars on cc and pay the %. It all depends if it is worth it to you or not. I pay with AX and sometimes MC. All depends on what benefit there is for me.

However, even though you don't care most likely. You should know that if a merch runs too large of a transaction or runs too many transactions of a certain amount, their funds may be put on hold and it can take from a day or two to over a month before they get their funds. They can also be told by the bank to refund the charge and collect the funds by another means. That is the acquiring banks option.