Most retail businesses run their physical stores and online shops on two completely separate systems. Cashiers scan items on an offline till, while an e-commerce website sells from the same warehouse. The nightmare is inventory collision: an in-store shopper walks up to the counter with the exact last sneaker at the same second an online customer clicks buy.

I built GTS as a solo contractor to solve this under a unified Turborepo monorepo. It combines a Next.js 15 customer-facing web storefront with an internal tablet POS dashboard for floor cashiers. Both frontends communicate directly with atomic Supabase PostgreSQL transactions and Upstash Redis reservation locks, decrementing stock in real time so overselling physical inventory is mathematically impossible.

To deal with spotty Nigerian card networks, checkout reconciliation never relies on client redirects. Fulfillment runs strictly on cryptographically signed Paystack server webhooks, with all accounting calculated in exact kobo integers to prevent floating-point rounding errors. Custom JWT auth claims let Next.js Edge Middleware protect admin and cashier routes with zero database lookup latency.