Senior Full-Stack Engineer, Billing & Subscriptions

We sell seat-based plans, billed monthly or annually, and the money math behind every plan change has to be exactly right. We're hiring a senior engineer to own the subscription engine: a pure core that takes the current subscription, a change request, and a point in time, and returns the proration plus the new state. This is subscription and proration logic, not invoice rendering or usage metering.

What you'll do:

• Get proration correct across mid-cycle upgrades and downgrades, seat changes, trial conversions, pauses, and cancellations with refunds
• Make the real judgment calls: how you count days, how you round, and when a downgrade is a credit versus a refund
• Guard against illegal state transitions so a subscription can't end up somewhere it shouldn't
• Keep everything deterministic, with money in decimal and time passed in rather than read from the clock

What we're looking for:

• Strong C# and .NET, with real care around decimal money and rounding
• Experience with state machines and date math
• Enough restraint to keep this a proration engine instead of growing it into a full billing system

Nice to have:

Background in billing or subscriptions, and comfortable designing an immutable, functional core