Hi! I'm the author of this blogpost. Thanks for posting it! Happy to answer any questions that anyone may have. If it's of interest, we also published a 2022 version last year, so you can see how things changed: https://getwaitlist.com/blog/solo-dev-startup-stack

TL;DR almost all transactional email services are very expensive, and they're driving most of our increased costs as we scale. Almost all other services we use have good, free/low-cost plans that don't break the bank as we grow.