Chargify

Chargify (now merged into Maxio) is an incredibly powerful, fiercely specialized titan that mathematically dominated the "Developer-Friendly B2B Billing" market for over a decade. It engineered a terrifyingly robust API. It is the absolute weapon of choice for technically sophisticated SaaS companies who mathematically demand a highly flexible, event-driven billing API that can physically handle the full complexity of usage-based and hybrid pricing models programmatically.