I wrote a guide to integrating Stripe payments into Rails applications that expands greatly upon the 10-minute quick-starts[1]. It launched a little over a month ago and has generated more than $10k. I'm expecting that to drop quite a bit going forward but I'm still hoping for ~$500 in monthly revenue.
I set up a mailing list right at the beginning (there's a signup link way at the bottom of the landing page, need to make that more prominent) that generated quite a bit of interest. My website by itself generates a decent amount of traffic as well. It's also been featured in a few podcasts and email newsletters which generate traffic and sales.
I've been considering selling on other sites (maybe Amazon) and/or doing some limited affiliate deals, but I haven't put any serious work into it yet. What sites were you thinking of?
[1]: https://www.petekeen.net/mastering-modern-payments