Landing a first major customer is crucial for startups, as it showcases their capability to handle significant clients and builds trust with other potential customers. Lago, an open-source platform, achieved this by securing Mistral AI as a client within a remarkably short timeframe of 17 days from the first demo to being live in production. This success was facilitated by a chance engagement through a post on Hacker News, leading to discussions about Mistral's billing needs and Lago's flexible, open-source nature, which allowed for trust and quick adaptation. Lago's Parisian roots and openness to collaboration, including integrating various payment vendors and making its roadmap public and participative, further cemented this trust. This collaboration not only accelerated Lago's business growth and helped in raising Series A funding but also emphasized the importance of prioritizing customer relationships and adapting product strategies to meet customer demands while maintaining the company's vision.