The article says he's tracking when recipients open emails and click links. So for every email one of his users sends there's some potential network traffic and data to be stored. He was probably able to offer the service for free originally because those features weren't added until after the subscription service launched. His AWS bill before adding those features was probably a few hundred dollars per month at the most. He also mentioned he is financially secure from selling a previous spam tool he created, so he could afford to run it for free for a while.