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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
> 2. How much can one reasonably expect to earn per user per month or year in your experience? (My extension is by no means mission critical but it solves a (simple) problem rather elegantly IMHO).
There's no way to tell for sure! It really depends on the value the extension provides, your market, exposure and reviews on the Chrome store, etc.
I can say that one of the extensions that uses ExtensionPay is a really simple utility that allows switching between recently used tabs in Chrome (https://chrome.google.com/webstore/detail/alt-%20-q-switch-r...) but users love it and it makes a surprising amount of money!
But all that is ultimately speculative for your extension. One reason I made ExtensionPay was to lower the barrier to making paid extensions enough so instead of speculating you could just try it out and see what happens!
> 1. Is there a socially acceptable path to go from free to non-free?
From what I've seen there will be a very loud but very small minority of users that will get angry and leave bad reviews. I think if you try to make these users happy (maybe by giving them free access?) and let the others know why you're asking them to pay then it will go alright. Here's what I wrote about charging for a previously-free JavaScript library: https://github.com/Glench/fuzzyset.js#license