Senior Next.js / TypeScript developer: help scale a Shopify checkout platform

We run Revio, a hosted, branded checkout for Shopify stores. Merchants add one script to their store. Their shoppers then check out on our faster, customisable page, with a drag-and-drop builder, order bumps, post-purchase offers, A/B tests, Google Pay/Apple Pay and ad-platform tracking. It is live and processing real orders. We are a small team, and I am looking for a strong developer to work with me on the next stage. Our stack Next.js 16 (App Router, server components), React, TypeScript, Tailwind Prisma 7 on Neon Postgres, deployed on Vercel Shopify Admin and Storefront GraphQL APIs, plus a storefront embed script Whop Elements for payments (card, Google Pay, Apple Pay, local methods), and webhooks Google Ads, Meta CAPI and TikTok tracking, server-side Playwright and Vitest for testing What you would work on (first 3 months) Checkout speed and conversion. Benchmark against the best checkouts in the market, and cut time to a usable page and to payment-ready. We already have a lab benchmark harness. We need someone who enjoys shaving 100 ms off real devices. Experimentation. A proper old-versus-new test framework with sticky cookies, per-variant reporting and stop rules, so we can prove what raises conversion instead of guessing. Tracking you can trust. Server-side conversion events (Google, Meta, TikTok), deduplication, funnel analytics (“opened, filled, paid”) and clean session and bot filtering. Payments and edge cases. Wallet buttons across Safari, in-app browsers (Instagram, Facebook, Google app) and Android. Discount stacking with Shopify’s combination rules. Multi-currency. Free and zero-total orders. Subscriptions. Reliability. Monitoring, alerts, safer rollouts (feature flags, kill switches) and test coverage on the money paths. Product features. Post-purchase offers, order-bump and upsell logic, new checkout blocks, and a merchant dashboard. You are a good fit if you Have 5+ years of professional TypeScript and React, and have shipped production Next.js (App Router) apps Are comfortable with Postgres and Prisma (or a similar ORM), including safe migrations on a live database Have built or integrated payment flows (Stripe, Adyen, Whop or similar) and understand webhooks, idempotency and failure modes Have worked with the Shopify APIs (Admin or Storefront GraphQL), or can learn them fast Care about web performance (Core Web Vitals, hydration, bundle size, caching) Test your work in a real browser, including Safari and mobile, and report honestly what you did and did not verify Communicate clearly in English and work independently Nice to have Experience with conversion rate optimisation or A/B testing infrastructure Ads tracking (Google gtag and enhanced conversions, Meta Conversions API, TikTok Events API) Vercel, edge and caching knowledge, and Playwright or visual regression testing Experience in e-commerce or a checkout, cart or payments product How we work Async-first, with short written updates. We use Git and small, reviewed pull requests. Production changes go out in small steps, with a way to roll back quickly. Contract, 15 to 25 hours a week to start, with room to grow. Time zone overlap of 3+ hours with the Netherlands (CET) is a plus. Rate: [your range]. We start with a small paid test task (about 1 to 2 days), then a longer engagement if it goes well. To apply, please answer: Describe a payment or checkout flow you built or fixed. What went wrong in production, and how did you find it? How would you measure and improve the load time of a server-rendered checkout page on a mid-range phone? How would you set up an A/B test on a low-traffic site, and how do you decide when to stop it? Link to your best work, such as a repo, product or write-up. And come up with improvements for our platform (www.revio-checkout.com)