I've thought about this too, as I walk past some nice cars at work everyday that look like they're washed by kids at a fundraiser. As with Jim's concerns, I wouldn't be leaving a card to offer my cleaning services or even include any of my contact info, but rather just to raise awareness of Adam's products. I get a DVD with almost every order because I give them to friends, would it be spammy to leave one on a random windshield?