My project is called reMarkable Connection Utility (RCU), and it makes about $2.1k MRR, selling ~200 copies/month. Some months are worse, some are better.
I've never purchased any advertisement. Its users advocate for it through online forums. That genuine attestation, to its quality and also to me as a person/programmer, helps it sell. That its users, too, are free to share their copy with friends helps people encounter the program who otherwise would not have purchased forthright.
I sell it from my personal website with links to PayPal and Stripe. The return URL is its download page. No JavaScript anywhere. I provide the entire Git history of the project, as well as binaries for all major distros of GNU/Linux, FreeBSD, Windows, and macOS. I also provide two mailing lists -- one for all customers to be informed of updates, and another for customers to talk with each other, see my latest code patches, and talk about its general development and the reMarkable tablet.