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Retention - if you build something people want/use AND come back and use often, then you can usually figure out a business model to make it work (if there's a big enough market).
We've done something similar, but by delivering pizzas. You can get them talking about you before you come in if you have it delivered, wrapped in gift paper, with a mysterious, clever, anonymous note complementing their pipes. Then drop in and ask if they liked the poster. It will probably go...