Build a single checkout page for a small online store. The page renders a cart of line items, each showing a product name, a unit price, and a quantity the shopper can edit. Alongside the cart, an order summary panel breaks the charge down into subtotal, shipping, tax, any applied discount, and a final total. A promo code field lets the shopper enter a code and apply it, after which the summary reflects the result. A confirm-order button completes the purchase and moves the page into a clear order-confirmation state.

The interface must keep the summary consistent with the cart at all times: when quantities change or a code is applied, the subtotal, shipping, tax, discount, and total recompute and display correctly. Promo codes are checked against a known set of offers, and an accepted code reduces what the shopper pays. The confirm action completes the purchase and lands the shopper on a confirmation that reflects the order they actually placed.

Treat the example below as illustrative of the happy path only. Your spec should describe the full checkout behavior precisely enough that an agent can build a correct, usable store, including the pricing rules, how the summary stays in sync as inputs change, and how the page behaves when shoppers do the unexpected.