A best bet is the Black Glossy Matte Solid Hard TPU Case for iPhone 5C, model no. 5650-Black-iph5c, for $1.99 with free shipping. That's the lowest total price we could find by $4. It's available in nine colors.
Of note, Total Defense Premium Internet Security must be activated before the rebate can be redeemed, which involves providing your credit card information. However, you can cancel the service at any time within the first year without incurring any additional charges.