At the checkout screen there will be a box to enter discount codes, don’t forget to click SET.
Here are some discount codes to use:
DISCOUNT10 is 10% off 2-9 books
DISCOUNT20 is 20% off 10-49 books
DISCOUNT30 is 30% off 50-99 books
DISCOUNT40 is 40% off 200-499 books
DISCOUNT50 is 50% off 500 books or more