I'm being quoted at $500 over invoice on an Enclave, but the saleman is telling me that my invoice price (from Consumer Reports) is wrong because GM passes on a 2.5% advertising fee (National & Regional). He claims there's nothing they can do about that because that's what they get charged by GM when selling that vehicle. Is this on the up and up? I know I could negotiate with a dealer trying to pass on their own advertising costs to me, but is he snowballing me with this GM-administered fee, or is it really the case? Seems fishy. Please help.