Amazon offers Prime members the Nyko Uboost Extended Battery for Nintendo Wii U in Black for $10.97 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $4.98.) That's the lowest total price we could find by $8. It works in conjunction with the GamePad battery increasing battery life by up to double and features a built-in stand.
Although this item is temporarily out of stock, it can still be ordered at this price for delivery when available.