I built an interactive Music Theory course 8 years ago over a winter break and it continues to bring in enough to pay my rent each month.
I just thought there had to be a more intuitive way to learn music theory than the very boring and jargon-heavy alternatives.
It uses Tone.js to include little interactive pianos, guitars, and other demos.
I've done no marketing, it hit the HN front page for a day, and after that initial spike in traffic has been fairly consistent over the past 8 years.
It uses Stripe for payments and for the first few years it was only Stripe. 3 years in I decided to add PayPal support... revenue doubled overnight, mostly from international customers.
https://www.lightnote.co/