---id: cartstype: conceptowner: justincomponent: storefront-apidescribes:  - src/carts.ts  - src/money.ts  - migrations/0002_carts.sqlreviewed: 2026-10-08---# CartsA 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]]).## Prices and totalsEach 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-in-whole-cents]]).