There's been at least one mention about 20% off, but here's a link to their ad for the 20% off coupon and they'll take paper copies of the ad or copy the number off your phone screen.
http://www.harborfreight.com/#
Also, you can get a free item (see ad, changes by the week), just make sure you grab the free item before you get in line. They are usually not right by the cash registers at the store I go to.