My idea...give the stickers to their customers.
That way, when I spill something, I put the sticker on myself. I feel better that it won't get missed by the dry cleaners and odds are, I'll go back to the same dry cleaner.
(Of course, any dry cleaner can see the "stain" sticker, but only one of them will give me more.)
It was enough of a good idea to get 2 shirts cleaned for free 🙂