When the cart is saved, Sprwt creates one Grab&Go order line per location on the delivery date and, for every meal in it, checks the location's on-hand stock for that meal and portion. If the stock is 0, the whole checkout fails with an Update Error message - typically because someone else bought the last unit between the customer adding it and paying, or because stock was removed manually in the meantime. If stock is available it records a Remove movement (reason Checkout) and lowers the stock by the ordered quantity, then saves the meals. Ask the customer to refresh the Grab&Go page: sold-out meals disappear from the list and quantity steppers are capped at the live stock.