I hope I'm not the last person on the planet to find out that Amazon has coupons. The easiest way to find them is by typing in "All Coupons" in the search box on Amazon.com. Once you "clip" the coupon it is automatically applied provided your purchase meets the coupon specifications.
Also, if you don't know about Amazon Mom, you need to sign up for that too. It gets you 15% off, certain items including diapers and wipes.
Last, but not least if you repeatedly buy something like diapers and wipes sign up for Subscribe and Save and get an additional 15% off.
Collect all the savings you can!