Senior Next.js Developer for Stripe Checkout

We run a golf tee box sponsorship business across 52 courses in the UK and US, with 800+ live sponsorships. Local businesses sponsor a hole on a 12-month rolling agreement, paid monthly or annually by Stripe subscription. Sponsorships are currently sold over email — sign, then pay, then artwork, across several days. People get stranded between stages. We're replacing that with a single-session self-checkout. The work A multi-step checkout: details and hole selection, terms acceptance, payment, artwork, digital pass. Built as a standalone app, served under the main site via path rewrite. The interesting parts: Concurrent inventory locking. Holes are finite and run at 86% utilisation. Two people must not be able to buy the same hole. Multi-item subscriptions. One subscription per order, one subscription item per hole, each independently cancellable, with proration on add and remove. Future-dated first charges. Holes sold before course installation take card details now and charge months later — which means off-session mandates with SCA completed at setup. Volume pricing calculated on a marginal ladder then blended evenly across line items. UK and US, GBP and USD, with VAT and US sales tax. You will be given a complete 18-section technical specification. Commercial decisions are settled. We are not asking you to design the business logic — we are asking you to build it correctly. Required -Next.js (App Router) and TypeScript, deployed on Vercel -Stripe Billing in production — subscriptions, subscription items, proration, Payment Element, Stripe Tax -Webhook handling done properly — idempotency, replay, out-of-order delivery -Concurrency control on shared inventory -Airtable API or comparable external data source as system of record -Mobile-first. Almost all our traffic is Meta ads on phones. Mobile Safari is the primary target, not an afterthought. Nice to have: -Off-session mandates with SCA at setup -Meta Conversions API, server-side -Apple/Google Wallet pass generation How this will run -Short call -Paid two-day trial task — a real piece of the build, most likely the locking mechanism or the webhook handler -Milestone-based engagement with review gates -Independent senior code review of the payment path before launch (arranged by us — please be comfortable with this) Please answer in your application 1. Link me to a live product you built with recurring subscriptions. 2. How do you handle webhook idempotency and out-of-order delivery? 3. How would you stop two customers buying the same inventory item at the same moment? 4. Tell me about a payment bug you shipped, and how you found it. Question 4 matters most. Everyone who has shipped billing at volume has one. Non-negotiable We own the code outright and can host it anywhere. No proprietary platforms, no lock-in.