Conventional billing systems often fall short for companies with complex billing needs, such as infrastructure and API-based businesses, as they require cumbersome custom scripts and logic to handle detailed invoices and specific charges, leading to inefficiencies and reliance on engineering resources. This complexity results in billing systems that mainly generate PDFs without offering real-time revenue insights. To address these issues, Lago introduced custom SQL expressions, allowing businesses to define and compute intricate billing metrics directly within the system. This feature enables companies to send raw event data to Lago, which then calculates and aggregates it into invoices, reducing the burden on users to develop their own billing logic. By supporting complex billable metrics, such as charging per user per repository, Lago provides a more flexible and efficient billing solution that can integrate with popular data sources like AWS, GCP, and MongoDB.