If you are talking a hand-wash, what would you have to pay locally to have that done at a GOOD carwash or detailer ? Do you specify any special treatment - like hand dry with microfibre only ? If so - then you should probably tip at LEAST what you would tip at the place you would have to pay for it. 20 % maybe for good service ? Sure the BMW Dealer is paying for the car wash, but just like when you get food at a restuarant on a coupon, you should tip like you were paying full price. (That is my Dear Abby / Emily Post moment for today
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