The Extra Wallet, well, its name speaks for itself. If you own of these, the capacity of your current wallet will be increased further–by 300 Rupees. Essentially, they’re wallet extensions.
For example, if you have the Small Wallet, which can carry 300 Rupees, and you obtain an Extra Wallet, you can store 600 Rupees. If you have a Medium Wallet and two Extra Wallets (they can stack), you can store 500 + 300 + 300 = 1100 Rupees.