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Payback is a positive status effect in Paper Mario: The Thousand-Year Door. It is symbolized by a rebounding Pink Arrow. When the character is inflicted, all direct attackers will receive half the damage the character receives, rounded down. Mario can only receive this status effect by using a Spite Pouch, which lasts for three turns. Wearing the Return Postage badge does "send back damage", but does not give the status effect. The two can stack, sending all the damage back to the attacker. Admiral Bobbery's Hold Fast move inflicts himself with it.
Having Payback on oneself, having Return Postage equipped, or having both on the player does not allow the player to directly attack foes who have Payback without getting hurt. Koops also gets hurt if he attacks a foe with Payback. This makes it different than electrified in those aspects.