I wouldn't bother with grocery stores. Fewer eyes will notice a Post It among a hundred thousand products and shelf tags. Also, unlike the price/gallon, the weight of a product may have changed (like ice cream) and it might seem to be a comment on a particular brand.
Those stores are also regulated regarding pricing, shelf tags and so forth. Refrigerated shelves are specifically designed circulate air assuming which a Post It may interfere with. Also it gives minimum wage clerks one more thing to do in addition to collecting, sorting, returning unwanted stock left in random places, cleaning, restocking and a myriad of other things.
Best to stick with the gas pumps.