I'm due for renewal on my Amazon Prime membership today which means that I've had it for a year. In retrospect, it was worth every penny, certainly to me and probably to Amazon.

I got 2 day shipping at no cost for almost any item I bought directly from Amazon.com (though not from the other sellers they host).

On the Amazon.com side, they got more business from me as shipping costs were never an issue and I knew I'd get the products in a couple business days which is about how long it take me to get up the energy to get to a physical store.
Considering the membership cost in Amazon Prime, I paid under $1 per order for 2 day shipping this last year.

Yeah, that's a lot of orders, but when I can easily find what I'm looking for and a single click gets it delivered to me quickly and free, where else would I shop?