Why billing systems are a nightmare for engineers

The challenges of automating billing systems for growing companies are highlighted through experiences from firms like Qonto, Pleo, and Algolia, which underscore the complexity that arises as pricing models evolve from simple subscription plans to intricate hybrid models incorporating both subscription and usage-based charges. Key issues include handling billing cycles, managing upgrades and downgrades, calculating usage-based fees, ensuring idempotency to prevent double charges, navigating cash collection intricacies, and dealing with diverse tax regulations. These complexities necessitate significant engineering investment and often lead firms to consider whether to build in-house systems or rely on specialized third-party billing platforms. The text advocates for an open-source billing API approach to offer flexibility and customization in addressing these challenges.