From the Super Mario Wiki
Flipside Tokens are special Rubee-like tokens that can be won by playing minigames in the Flipside Arcade in Super Paper Mario. They are similar to Pianta Tokens from Paper Mario: The Thousand-Year Door.
By playing the minigames in the arcade, Flipside Tokens can be won and exchanged for rare prizes. Flipside Tokens can be bought for three Coins each, and three of the four minigames cost ten Flipside Tokens to play. The secret game, accessed with a Golden Card, costs twenty Flipside Tokens to play. The player will earn one Flipside Token for every 10,000 points they score in any minigame.