The sole reason I do not have Amazon prime is because I know I will spend too much. Haven't had it in years.
They're probably breaking even on prime memberships or even losing moneyhaving to pay FedEx and UPS to express shipeverything.
Hence them being in the process opening up their own shipping company
The real money is in the fact that people spend much MUCH more on Amazon knowing that something will be there in only two days. Prime is a means to and end. A very very successful one. Remember users are the real product at the end of the day.
Edit: this article says Amazon Web Services is where the profit comes from. They are in fact still losing money on their e commerce but market share > profit for now. Gotta stiffle the competition by any means
All of Amazon's 2017 operating income comes from AWS | ZDNet