Promo codes, second pass

Hi, I run an e-commerce website specializing in promotional merchandise for the Australian market. THE PROBLEM Discount codes work, but the system has grown unevenly. Three separate places in the application redeem a code, and they no longer agree with one another. On the main storefront checkout, the discount amount is read from the browser and never recalculated on the server. That same path also fails to record the redemption, so the usage limits configured in the admin panel are not enforced there. I already know about both issues. I am not asking anyone to find them; I am describing them because fixing them is the job. FIRST PIECE OF WORK — PAID TRIAL (200$, fixed price) 1. Stop /api/checkout from accepting a discount amount from the client, and recalculate it on the server using the existing shared evaluation function. 2. Make redemption atomic, so that a code limited to ten uses cannot reach twelve when requests arrive simultaneously. 3. Increment the usage count on that path, so the figures shown in the admin panel are accurate. 4. Add enough automated tests to prove the three points above. There is no test suite at present, so establishing a minimal one forms part of this work. Please keep the trial to those four items. ONGOING WORK AFTER THE TRIAL If the trial goes well, I would like to continue on a part-time basis, roughly fifteen hours a week, working through the rest of the roadmap: GROUND RULES No price, discount, or total is ever taken from the client. Existing live codes must keep working through any migration, without manual intervention in the admin panel. GST is 10% and applies to the discounted amount. If that calculation is wrong, the invoices are wrong. None of this work should change the appearance of any page. I will share the full brief and repository access once we have agreed to work together.