Use a blank space on said sticker to Sharpie on customer name/order number and/or date, this can serve a a warranty period without the need to produce a receipt that could be long lost. It will also give people less reason to remove it or be annoyed they have had something added for no reason other than pure marketing BS.
I feel like this is probably the best option so far to be honest.
I guess the other thing is how much you feel you need to repeat custom, if you're doing a good job then that customer comes back anyway as they already know about you, and no-one else would ever see it anyway?