From the Super Mario Wiki
A Courage Shell is an item found in Paper Mario: The Thousand-Year Door and Super Paper Mario. In Paper Mario: The Thousand Year Door, using one can boost Mario's or his partner's Defense by 3 points for two turns. Courage Shells can be bought in Petalburg and can also be stolen from Koopa Paratroopas. They can also be cooked up by Zess T. by cooking up a Mr. Softener and Point Swap together.
In Super Paper Mario, it reduces damage players take by half. To make it more effective, players have to play a Minigame similar to Koops' Shell Shield. The effect can last anywhere from 25 to 40 seconds. In this game, if one were to give a Shell Shock to Saffron, she would bake a Courage Shell. However, if one gives the Courage Shell to Saffron, she will bake a Mistake. Courage Shells can be purchased from Howzit's shop in Flipside for 10 coins. Various enemies can also drop Courage Shells.
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