Then yeah, just call it billing as a service.
I just dislike the wording of it all. "You don't have to do the billing in the backend, you can do it on the frontend". As if the frontend and the backend were mythical realms fighting for control.
We went from PHP generated websites, to PHP backend generation bad, use "client-side rendering" and "single-page applications", as the shiny new thing.
Then we went to Server Side Rendering (SSR), as a new thing. What do you mean a new thing? You're just doing in Node what everyone was doing in PHP 15 years ago.
And that's fine!
Not every idea has to be revolutionary, not everything has to be "the future", or "the only way".
You've found a service that does billing for you without having to integrate it into your backend. Great! Amazing! Good for you! I'm sure it's of interest to a lot of people!
But why can't you just, you know, say it like that?
This is just billing as a service.