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...