You can use Plugins/testharness.html to test out the WebMonetization.jslib https://docs.unity3d.com/Manual/webgl-interactingwithbrowserscripting.html. Look at the javascript console for details.
Download node.js
npm i -g md-to-pdf
md-to-pdf README.md
md-to-pdf README.md Assets/WebMonetization/README.pdf
Try to use oldest version of Unity available (currently 2017.1) that makes it compatible w/ more installs.
In Unity, Assets => Export Package => (add the web monetization folder). Save to /Releases with a new version number.
Use AssetStoreTools to actually release to the Unity asset store. Contact support (https://simmer.io/support) if you want to have me do this.