Since this is the right crowd, for those who make money with your extension, how do you do it? I'm contemplating building an API for license checking and a simple webapp for registration / subscription management, but if anybody's using the off-the-shelf options I'd be interested to hear about your experience.

I've heard good things about ExtensionPay[1]. It didn't have what I needed at the time so I hand rolled my own on Firebase using a Stripe integration component[2]. As you probably know the issue with extensions is that the whole codebase is exposed so you need some kind of back end if you're handling payments and associated secret stores.

[1] https://extensionpay.com/ [2] https://extensions.dev/extensions/invertase/firestore-stripe...

I'm in the process of adding payments as well and I'm starting things off with Stripe. Specifically, their payment links and subscriptions features. Longer term, I may switch, and am building things in a way that migration will be easy.