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 :heelclick
Of all the marketing tools that I have used; newspaper, internet (craigslist, google ads, etc) signage, etc... This is the most effective:

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 :heelclick
Of all the marketing tools that I have used; newspaper, internet (craigslist, google ads, etc) signage, etc... This is the most effective:
