system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does Amazon Prime end up being profitable for Amazon?

I have been using Amazon Prime for almost 2 years now and love it.  I just do not understand how they can be making money on it.  I often order small things, under $20 on a Wednesday and the two day obligation falls on a Saturday. So they FedEx Saturday deliver it, or just overnight it to get there on Friday, a day early. I also recently moved and ordered a washer and dryer from them and the prices were slightly cheaper than the department stores, and it was delivered free via a freight service (JB Hunt) . I realize they probably negotiate contracts with FedEx and UPS, but I still have not clue how this works out for them when they only get 80 bucks a year from me.
gpt
It encourages you to keep buying from them. Sure, they'll lose out on shipping costs, but in theory, you might be more prone to purchase from Amazon, so you feel like you're getting your money's worth from your membership.