Discount code: DADSCASENTEE. Good for 50% off shirts and cases on entire Zazzle site.
Free shipping with free trial of Zazzle Black (sign up for it then cancel, or get unlimited shipping for $9.95 for a year).
Not every dad can be World's Best or Number 1. Some just get the job done without screwing up too badly. Also available in adult sizes for surly teen-agers or passive-aggressive adults.
And yeah, it's just text on a shirt. But it's cheaper than therapy.