Did you know that Amazon has Coupon Codes? Mostly for grocery items and personal care/beauty items, there are also coupon codes for Movies & TV and even Books. These are easy to use – just click the coupon on the product page before you order (which you can do then or any time before the coupon expires, once you have clicked to add it to your checkout); there are no promotion codes to enter for these.
Kobo often runs a Trivia Contest and gives away additional coupon codes (and Kobo eReader, so be sure to use a real email address).
Current coupon codes for Kobo Books: