Well, they're graphic novels and she sells them for 9.99 with a paperback around 17, I think. But she made most of that 60 million when Netflix optioned her books for a series. And Scholastic picked her up, so I'm not sure we can call her self-published. She may have started that way, but when she trended, someone at Scholastic picked her up and pushed her hard. She writes LGBTQ+ YA (real YA, not NA posing as YA), so it was also a matter of the right time.
ETA: Oh, also, she shunned the gay YA trope of one of the characters dying and hit an under-served target: 13-15 boys who are growing up in a time when the frat-bro aesthetic is no longer on point.