Usage-based pricing (UBP) has become a prevalent model for SaaS and AI companies, driven by the need to match costs with customer usage and address subscription fatigue. This model links revenue directly to consumption and can be more beneficial for cloud infrastructure and API-driven software when usage correlates with customer value. Implementing UBP poses significant challenges for developers, requiring sophisticated billing systems capable of precise metering, flexible pricing logic, and robust automation to avoid revenue leakage and customer dissatisfaction. Engineers face hurdles like handling complex billing logic, managing plan changes, and ensuring idempotency to prevent duplicate charges. Lago is introduced as a solution to these challenges, offering an open-source, API-first billing platform that supports various pricing models, including pay-per-use, credit-based, percentage-based, outcome-based, and hybrid models, while allowing businesses to focus on product development rather than the intricacies of billing infrastructure.