Proving to the cashier that your books are supposed to be 40% off.
The folks at my local JoAnn fabrics are great. They are always friendly and helpful, despite not-so-great hours, not-so-great pay, and not-always-so-great customers.
When my book purchase rang up at full price, I excused myself from the checkout line to snap this photo of the merchandise display. It was proof enough for her to adjust the prices on my books. Yay iPhone and yay Cashier!