Audit & Fix Existing Self-Hosted Shopify Checkout

We need an experienced developer to audit and fix an existing self-hosted Shopify checkout built with Next.js, TypeScript, PostgreSQL and Whop payments. The work includes debugging checkout UI, autofill and validation, payment loading, idempotency, webhooks, Shopify order creation, shipping data and real browser checkout flows. This is an existing system, not a build from scratch. You must identify root causes, implement safe fixes and test the complete customer journey. If you do not have proven experience building custom self-hosted Shopify checkouts and payment integrations, please do not apply.