Note: the best defense against grin fucking is to call attention to it by casually asking the salesman why they are smiling.
You <after sipping your coffee>: Why are you smiling? That sounds like bad news to me.
Salesman <suddenly befuddled>: Oh... no reason. I guess I'm just in a good mood today.
You: Okay, well, since I never approved the additional costs, what discounts can you provide to bring us back closer to the original estimate?