Senior Software Engineer (Payments) (f/m/d)

We are Billie, the leading provider of Buy Now, Pay Later (BNPL) payment methods for businesses, offering B2B companies innovative digital payment services and modern checkout solutions. We are to create a new standard for business payments and have made it our mission to simplify the purchasing experience for all businesses making it a tool for growth. Our solutions are based on proprietary, machine-learning-supported risk models, fully digitized processes and a highly scalable tech platform. This makes us a deep-tech company building financial products, not the other way around. We love building simple and elegant solutions and we strive for automation and scalability. About the role: Billie is cracking the B2B finance world, and we are looking for a Senior Software Engineer for the Payments domain. The Payments domain handles the complex flows of identifying, allocating, and tracking money as it moves between merchants, buyers, investors, and Billie itself. We integrate with banks, generate payment instructions, and process bank statements. Our teams have a high degree of autonomy and make their own decisions about how to meet customers' needs. In this role, you will: - Design, develop, test, deploy, maintain, and improve the software your team owns, from new features to evolving existing services. - Understand, modify, and debug complex business flows across existing Kotlin and PHP systems, take part in incident response, and own RCA follow-ups. - Work with the Staff Engineer and the architecture group to turn the target architecture into incremental, well-scoped plans, and deliver them. - Write technical specifications for new features, covering the problem, the options, the trade-offs, the chosen design, and testing and rollout. Break them down into detailed tickets the team can deliver, and review specs written by others. - Use AI agentic coding tools in your daily work to explore code, draft implementations, write tests, and review changes, while staying fully accountable for what ships. Help the team use them well by sharing workflows, writing good context for agents, and contributing to Billie's shared AI skills. - Review code, mentor less experienced engineers, and unblock teammates so the whole team moves faster. - Represent your team with stakeholders to drive decisions, and help with interviewing. We expect a significant part of your daily work to be hands-on coding. What you’ll gain: - Deep domain expertise in B2B payments. You’ll work on real money flows: bank integrations, payment instructions, statement processing, and allocation logic. Few engineers get to see this domain up close. - Real ownership and autonomy. Our teams decide how to meet customers’ needs and run what they build, so your work has a direct, visible impact. - A front-row seat in AI-assisted engineering. You’ll work with agentic coding tools and spec-driven development every day, and help shape how Billie uses them through internal workshops and our shared AI skills. - Room to grow. We expect and support continuous professional development, with a clear path toward Staff-level responsibilities if that’s where you want to go. Who we are looking for: - You have solid practical engineering experience (5+ years). - You have proven experience putting Kotlin (ideally) or Java-based solutions built on Spring into production. - You have experience building and running microservices in cloud-based environments with high availability and reliability targets. - You have hands-on experience with AI agentic coding tools and spec-driven development on production work. You know when to trust their output and when to challenge it, and you write specs other engineers can build from. - You can argue for and against architectural decisions using clear trade-off analysis, and you hold strong opinions loosely. - You have worked in a product-focused environment. - You have a keen interest in understanding the world of b