The demo ranks prompt quality by public checks, hidden domain checks, UX/style, and prompt efficiency. Runtime is intentionally not part of scoring.

Use integer-cents totals; normalize promos; cap discounts; pre-discount free shipping; enforce stock, double-submit, loading/error, ARIA, and mobile behavior.
Use a spec with states, acceptance tests, promo validation, quantity limits, cents-safe totals, and clear mobile/error behavior.
Build an ecommerce checkout web app with cart items, quantity changes, promo code, totals, and confirmation. Make it look nice.