I'm big on nutritional supplements. That's really the bulk of it. Amazon has the selection and prices I'm looking for. If you're older and your body is cranky, definitely look into things like the following:
Between me and my girlfriend we spend a couple hundred dollars a month on various supplements, which would easily cost almost twice as much retail for the same quality stuff.
I love Costco too. You can't go wrong with them for a lot of stuff. It's much easier for me to cost/quality compare online for supplements vs Costco. I also don't have to buy in bulk w/ Amazon while getting approximately the same per unit cost.
I'm all about Costco for food stuffs though. I've just discovered how great their clothes is to. I'm also lazy so having stuff arrive at my door is a huge bonus on Amazon, but I do love Costco too.