As long as you sign it AND the dealer signs it, then you've got an official pricing contract. If they sign an itemized purchase contract with you, then that's it. It's a contract. You can have them add on there, "This vehicle will be sold at MSRP, no additional dealer markup" if you want.
They are pretty easy about the pricing. I call up once a year and simply say "I saw the promo on your website and I'd like that applied to my account" and they just do it. Otherwise you're paying like $22/month
They've got a 3 months for $1 promo for the streaming for new subscribers.