Carts

A cart is a row in carts and its lines in cart_items. Anyone can create one with POST /carts; the id is a random UUID, and knowing it is what gives access to the cart.

Setting quantities

PUT /carts/:id/items/:product with { "quantity": n } sets how many of a product the cart holds. It sets rather than adds, so a client that retries after a timeout does not double the quantity. Zero, or DELETE on the same path, removes the line.

The limits:

• at most 10 of one product and 50 different products per cart;
• never more than the product's current stock (the error says how many are left);
• only active products, and only while the cart is open.

Setting a quantity checks stock but does not hold it. Stock is only taken when the cart is checked out (Checkout, 0004 Take stock at checkout, not when items go in the cart).

Prices and totals

Each line stores unit_price_cents, the product's price when the quantity was last set. totals() in src/money.ts adds the lines and a flat shipping rate of $5.95 for subtotals under $50; from $50 shipping is free. All of it is whole cents (0003 Money is whole cents).