Clone Slack's fair per-seat pricing in a simple way

Lago is an open-source billing API designed to help engineers implement billing engines quickly, particularly for companies adopting complex per-seat pricing models like those used by Notion or Slack. This model ensures customers are billed only for active members based on actual usage, with charges prorated according to the number of days members are active during a billing period. Lago simplifies this process with its persistent billable metrics feature, enabling developers to replicate such pricing systems efficiently. The setup involves configuring a billing engine, creating persistent billable metrics, and establishing a price plan using the API, with seat charges calculated monthly at €8.25 per active user. Through the API, companies can create customers, assign plans, and manage subscriptions, while events can be sent to track user additions or removals and ensure accurate prorated billing. The process culminates in comprehensive PDF invoices that detail seat usage and charges, demonstrating the ease and maintainability of Lago's solution for implementing per-seat billing systems.