What is the application stack you are using? Something like Laravel or Symfony (PHP) or Django (Python) would come with auth stuff already and it wouldn't take

What is the application stack you are using? Something like Laravel or Symfony (PHP) or Django (Python) would come with auth stuff already and it wouldn't take too much effort to make it API-oriented. Connect it to something like Stripe, deploy it on e.g. Heroku or so (I personally use AWS Elastic Beanstalk for almost all my projects, mostly because I feel most comfortable there) and you should be almost there. I have 2 side projects where I'm the solo developer, one runs on Symfony, the invoices are generated, but after they're sent, I need to manually check them (but each invoice is 100-200 US$, so it's ok). A second project runs on Django and is subscription based, here the invoices are higher in volume but only 5 US$ each. There billing goes via Stripe (among others) and is handled automatically. Check out https://www.indiehackers.com https://www.indiehackers.com as well for some inspiration.