I love buying from Amazon.com. Like Jaye and JM said, you get 15% off your order, and currently you actually get 30% off your order (if you set up a monthly delivery). And even before the 15 or 30% off, their diapers are cheaper than Costco's - and free 2 day shipping! There's also a 20% off Amazon coupon in this month's Parents magazine that can be stacked with the 30% off for even more savings. I think the coupon expires Aug. 31.
I also use Swagbucks.com as my search engine, and earn points through their site that I use to buy Amazon gift cards. I've been using Swagbucks for almost a year, and I've earned about $30 in gift cards so far. (You can use the points for other types of gift cards too, but for us the Amazon cards are the most valuable.)
Before the birth of my third son in April, I was addicted to couponing. But since then I just don't have the time, and this deal from Amazon has been a Godsend.