This deal has ended.
Amazon now has the GE Ultrasonic Sensor Parking Alert System on sale for just $14.13. This installs in your garage and sends a signal to the sensor in your vehicle to make sure you park at the exact desired length from the wall (no more hanging tennis balls). It's received 4.7 out of 5 stars from its first eight reviewers. Shipping is free for orders over $25 and there's no sales tax (except KS, KY, ND, NY, WA).