Note in a bottle

From Wild World Wiki

Note in a bottle is an item which you can buy to send a message to a random person through Tag Mode. These work like regular mail (and you can even attach gifts), except that you have to be near a river/sea/lake that's not a pond (next to it) and select the command "Throw in" to chuck it in the water, where it will then drift out of view and into the open ocean, where it will land on another person's shores through Tag Mode.

You can receive these kind of messages through Tag Mode too, and when you do, it will wash up on shore where the shells appear. However, random messages that are not sent by any particular real person will sometimes appear too. These range from odd writings to coupons and sometimes the ingredient list to a vitamin bottle. They are all just fun messages that you find, however, and have no reward other than a good laugh. So don't bother saving them if you need to make space.