Groupon offers the refurbished Mophie Juice Pack Air 1,700mAh Charging Case for iPhone 5 / 5s in Black (pictured) or White, for $24.99 with free shipping. That's tied with last week's mention as the best price we've seen for this case. (It's also $75 under the lowest price we could find for a new unit.) This 1,700mAh battery pack provides up to 10 hours of talk time or up to eight hours on the Internet.
A 90-day warranty applies, although it's unclear who backs it.