What the backorder shipment costs. Free shipping is always judged on the whole order, for every
shipment. fixed: amountIncVat, even when the order ships free. same-as-order: the
shipping method's price, free when the order ships free. free: nothing. The first shipment
always costs what its shipping method costs.
The store offers the choice (Settings → Order routing). When false, show nothing.
Something is missing and the cart can be split. When false, everything ships together.