The Extra Wallet is an item found exclusively in Skyward Sword. It is used to allow Link the ability to carry an additional 300 Rupees in his wallet, on top of the amount carried by his normal wallet. This is useful, as there are plenty of things that cost more than what Link's wallet can hold. These Extra Wallets can help Link collect enough Rupees. Each Extra Wallet only costs a mere 100 Rupees. It can be purchased in Beedle's Air Shop three times throughout the course of the game.
Link is able to purchase a total of three extra wallets, bringing his additional Rupee total to 900.