If you connect your Stripe account to Kable, you can enable automatic tax calculation in your Stripe dashboard, and invoices generated by Kable will be taxed by Stripe. As we add other payment partners, we will follow the same pattern. In the future, though, we do plan to add the option to calculate taxes directly within Kable.
Kable uses Stripe's Invoice API directly (we don't create Stripe subscriptions which would cost an extra 0.5%). You can set up Stripe Tax to work with Invoices, and I believe most of what you'd want happens automatically.
I may be wrong though -- please let me know if you've had a different experience!
Kable can be accessed through our Node and Python libraries (more languages on the way) or over HTTPS REST API. I will have to defer to your design sense about the best way to leverage these technologies with a GraphQL API today.
This means to record for billing, you would need to parse the AST and tag the parts as well as their associated cost in a query.
It's not an easy drop in, unfortunately.