EC Technology via Amazon offers its EC Technology Ultra-Slim Foldable Bluetooth Keyboard for $36.99. Coupon code "D5D9XPAX" drops it to $26.99. Plus, Prime members bag free shipping. (Non-members can spend $35 to bag free shipping; otherwise, it adds $8.22.) That's $43 off list and the lowest total price we could find. It features a lithium-ion battery and is compatible, with Windows, Android, and iOS devices.