From the Super Mario Wiki
The Platinum Card is an item found in Paper Mario: The Thousand-Year Door. It is found by completing the Trouble Center mission "I must have that book." when the player gives Toodles the Wrestling Mag. It can be given to Lahla to allow Mario to play the Boat Mode minigame at the Pianta Parlor. It adds the HP Plus P badge, Gold Bar x3, and the Money Money badge to the "Trade in Winnings" option of the Pianta Parlor.