Real Decision
Why billing evolved as an independent service rather than staying inside product logic.
Problem
Billing rules, subscription changes and financial workflows evolve differently from customer-facing product behavior. Keeping them inside one application increases coupling and slows both product and commercial changes.
Decision
Billing was isolated as its own service with explicit APIs and integration contracts so subscription logic, invoicing and plan enforcement could evolve independently from client and operational systems.
Why Not
Keep billing rules embedded in every product surface.
Let commercial workflows share the same release cycle as application features.