CTC Store offers Amazon Prime members the Rider Leather Wallet Case for iPhone 4 / 4S in Black for $19.25. Coupon code "9RIGOLNK" drops it to $8.25. With free shipping, that's $11 off list and the lowest total price we could find. (Non-members can spend $25 or more to qualify for free shipping; otherwise, shipping costs $5.11.) Coupon expires January 25.

Amazon Prime membership is $79 per year; however, Prime is free for college students for six months, and free for parents and caregivers for three months. (Note that currently there's a waitlist for Amazon Mom.)