Software Engineer

About this role

About Flex
Flex was founded to solve a simple but costly problem: making it easy for health and wellness brands to accept HSA and FSA payments online. For years, only giants like Amazon and Walmart could process these payments at checkout, leaving smaller merchants — and their customers — to deal with frustrating reimbursement paperwork.


Our technology verifies product eligibility in real time, handles split payments for eligible and non-eligible items, and can even issue Letters of Medical Necessity during checkout. With $150 billion in annual HSA/FSA spending potential, we’re giving brands the tools to unlock new revenue and making it easier for consumers to use the benefits they’ve earned.


About The Role
As one of the earliest engineering hires at Flex, you’ll shape the future of HSA/FSA payments. You’ll work across the stack to deliver seamless customer experiences, build powerful APIs, and design scalable infrastructure that powers our mission. This is a rare opportunity to work directly with leadership, customers, and cross-functional teams to set the foundation for a category-defining company.


What You’ll Do

• Contribute to building the highest-converting HSA/FSA checkout experience.
• Build simple, elegant APIs that make Flex easy to integrate.
• Design and develop scalable solutions for HSA/FSA payment infrastructure.
• Interact directly with customers to help define the roadmap.
• Work with technologies including Next.js, Rust, TypeScript, Postgres, Redis, and AWS.

Who You Are

• 3+ years of engineering experience at tech and product-driven companies.
• Proficient in TypeScript, React, and modern web technologies.
• Skilled in at least one backend language (we use Rust).
• Hands-on experience deploying to AWS infrastructure.
• Comfortable in dynamic, fast-paced startup environments.
• Prior startup experience (seed–Series B) strongly preferred.

Mindset

• Low ego — open to feedback and growth.
• Collaborative — thri...