Well, $10.70 with tax
Low tack white vinyl. Same size as the oil change reminder stickers. Got 54 of them for $10. Place them above the mirror so the driver sees it every time they look in the mirror, but out of the way enough to not be removed immediately
Of all the marketing tools that I have used; newspaper, internet (craigslist, google ads, etc) signage, etc... This is the most effective:
