Answer
Two things need to be true for CustomsValue to actually transfer from Customer Order Lines to Shipment Lines: the Customer Order Line's CustomsValueCurrency field also needs to be populated — CustomsValue alone isn't enough, since IFS only performs the transfer once a currency is set alongside it — and if you want both fields populated automatically rather than entered manually, you'll typically need a Custom Event plus a workflow that generates CustomsValue on the order line, plus a second workflow that populates CustomsValueCurrency on the order's Delivery section. Only once both fields are actually populated does the data flow through to the Shipment Lines as expected.