I think Amazon's case is a little more specific. People want trustworthy delivery, and they like getting a few larger packages rather than many small packages.

I think Amazon's case is a little more specific. People want trustworthy delivery, and they like getting a few larger packages rather than many small packages. I could buy a few items from non-Prime sellers with free shipping, but then I'm going to get a bunch of different boxes spread out over a few days due to different sellers and shipping speeds. If I pay a little extra per item I know that Amazon will ship my stuff quickly and it usually comes in a single package, unless the items in my order are unusually different from each other.