I second Amazon, when he gets here, I'm going to sign up for Amazon Mom and do the subscribe and save, when doing that ya save a little more. Plus free 2 day shipping.
FYI, you can't combine the subscribe and save discount with the two-day prime shipping. It's one or the other.
If you are okay with store-brand diapers, target brand diaper are cheapest by far (even cheaper than Costco). Also, I think Target ships free over $50 but they're always out of the size I need online.
Amazon IS great for other stuff, though. I get 3-packs of Burt's Bees shampoo and baby wash. Also things like puffs, baby oatmeal (we skipped rice cereal), diaper genie bag refills, etc. It's totally worth paying for prime and they do have good deal on the name brand diapers and wipes. Once we switched to Target brand for those things, though, we never had a problem. Same for formula - Target has great prices.