Groupon offers the refurbished Mophie Juice Pack Air 1,700mAh Charging Case for iPhone 5 / 5s in Red, White, or Black for $24.99 with free shipping. That's tied with last week's mention and the best price we've seen for this case. (It's a current price low for a refurb by $5). This 1,700mAh battery pack provides up to 10 hours of talk time or up to 8 hours on the Internet.
A 90-day warranty applies, although it's unclear who backs it.