You can't get something for nothing, but in this case you can get pretty close. Power Skin has a killer Cyber Monday promotion with its PoP'n selling for just a penny. You still have to pay shipping, but that's only $8 for basic US delivery. You're probably wondering what a PoP'n battery is. Well, it's kind of weird.
These batteries have an array of suction cups that stick to the back of your phone so you can continue to use it (somewhat) comfortably while it's charging. The price is usually $49.99 (the larger $80 iPhone 6 version isn't included in the promo), so you're saving big. Just enter promo code PENNY at checkout to get the deal.
These are 2000mAh li-ion cells with slightly different shapes/sizes for different phones. There's one for HTC phones (I guess because of the curved back), then one for Samsung, Motorola, and Sony. The PoP'n isn't meant to provide multiple charge cycles, but it lets you get a little boost when you need it. If that sounds good, get on it. This is only valid today.