Changes the cart's settings: fulfillment method and location, shipping and billing addresses (shipToId, billToId), payment method (paymentId; "new-card" to enter one at checkout), carrier account, shipping rate, coupon or referral code, store credit (useCredits, creditsDecision), purchase order number, note, requested date, subscription plan and frequency. Send only the fields that change. Any change invalidates the quote: read cart.get { quote: true } again before checkout.
Who may call it: A guest (X-Storefront-Guest), or a signed-in buyer (X-Storefront-Buyer) whose account is confirmed; an unconfirmed buyer is refused with 403 CONFIRMATION_REQUIRED.
Revision. Send expectedRevision, the revision of the cart you last received (cart.get or the previous write). The cart is written only when it still matches; otherwise the answer is 409 and you reload the cart before retrying. ownershipVersion, from the cart, pins the cart a guest or a claim acts on. A successful write answers the whole cart with its new revision.
