From the Super Mario Wiki
Return Postage is a badge in Paper Mario: The Thousand-Year Door. It is the reward found in a treasure chest after reaching the 100th floor of the Pit of 100 Trials and defeating Bonetail. If Mario equips it, enemies who come directly into contact with him while attacking are damaged by half of what they dealt to Mario. This applies to exactly what Mario received as damage, meaning that guarding could reduce the damage that the enemy takes. If the player sells Return Postage to the Lovely Howz of Badges and wishes to buy it back, it would sell for 999 coins, making it the most expensive item in the game. If Mario combines this badge with the use of the Payback ailment via Spite Pouch, all damage will be sent back to the attacker. However, if Mario has Return Postage and Zap Tap equipped, he will not be immune to enemies that drain HP or FP such as Swampires or Flower Fuzzies.
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