That part isn't. Not so much because we care about it being closed source (a bit), but more because there's little to no value in it being open sourced.
Billing is all based on Stripe, so its not super heavy by design. We basically just synchronize the data that's in Stripe. That's changing a bit as we work towards a new model, which requires a lot more internal systems to coordinate, but generally not too complex still.
Is the plan/user/payment management also open source?
I mean, if I were to start a small SaaS service written in Python, could I use your code to bootstrap the "boring parts" of it?
Thanks!