From reading this thread, it seems clear that dealerships cannot set a minimum or maximum payment amount for using AMEX (for example). Also, it's clear that if a dealership accepts the card in their service department, then they need to accept it in sales as well.

But what still isn't clear to me is whether a dealer can tack on an additional x% "processing fee" to cover what they'd pay to the card company for the transaction. Or, if they don't push it as a fee for using a credit card, it would be a "discount" for paying in cash. Could someone please clarify this? TIA. The prospect of 22k Starwood points is pretty intriguing...