The increased switching cost isn't by force, though. Customers are less likely to switch because of convenience -- you're paying for the conveniene which, I think, means that Amazon is providing a service that consumers want.
I don't know what the things I used in the dinner I just prepared costs. I only know roughly how much my weekly grocery bill comes to (it's all delivered - the order is auto-populated by Ocado's algorithms, so if I don't do anything I get a reasonable default set of products based on what I usually buy). And I like it that way.