Serverless products inherently require usage-based billing models. However, building such systems is notoriously difficult due to the need for flexibility, scalability, and audibility. WarpStream, a serverless product, partnered with Orb to offload hard problems like event ingestion, processing, and storage from their engineering organization. To implement a simple yet effective pricing model, WarpStream created a time series event structure that separates "events" from "metrics," allowing for easy addition of new dimensions in the future. The system also employs idempotency keys to prevent double billing and incorporates checkpointing to optimize progress tracking. For its serverless product, WarpStream implemented an in-memory two-tiered aggregation architecture to measure network ingress and egress fees incurred by transmitting data through cloud environments, resulting in negligible overhead and a simplified architecture. By partnering with Orb, WarpStream was able to implement this usage-based billing model efficiently, reducing operational burdens and costs.