When I had my car detailed, and paid with a credit card, the machine gave a option to add a tip. I was a little surprised. It would not have occurred to me to tip for this service, it cost enough already, and I never actually met the people who did the actual cleaning, but this line on the credit card machine made me wonder if I was "supposed" to. But I usually just tip servers, people who do personal body services (hair, nails), and movers (as I feel awful watching them sweat as they haul my heavy, awkward furniture around).
But as for the OP's question, I agree that if it's a service that is normally tipped, you do tip on the pre-discounted price, as with any coupon.