Accesoory 4 U via Amazon offers Prime members the Zotech Ultrathin Keyboard Cover for iPad Air for $9.99 with free shipping. (Non-members can spend $35 to qualify for free shipping; otherwise it adds $8.64.) That's the lowest total price we could find by $10. It features a built-in 800mAh rechargeable battery and pop-up stand, and doubles as a cover when closed.
Orders placed by 11:59 pm ET December 18 are guaranteed for Christmas delivery via Standard Shipping. (The deadline is December 22 for Prime 2-Day Shipping.)