CTCStore offers Amazon Prime members the Acase Betelgeuse 3D TPU Skin Case for iPhone 4 for $8.95. Coupon code "34E7NO5I" cuts it to $1.95. With free shipping, that's $7 off and the lowest total price we could find. (Non-members can get free shipping with orders of $25 or more; otherwise, shipping adds $5.05.) It claims to fit iPhone models from AT&T, Verizon, and Sprint. Coupon expires January 22.

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.)