From the Super Mario Wiki
The Smile
The Item Shopkeeper will give a bright smile to the player.

First appearance

Wario Land 4 (2001)
The Smile in the Item Shop menu.

The Smile appears in the Item Shop in the Game Boy Advance game Wario Land 4. It is the only thing in the Item Shop that is free, but it has no purpose whatsoever. It is a joke item, and all the player gets is a smile from the Item Shopkeeper. A sound effect also plays each time the player selects it.

All of the other items in the Item Shop are there to assist the player in defeating a boss. They all cost a certain amount of medals, which is the currency that is used in the game.