We need to update shipment state for all inventory units of a order here as well.

```php
public function xxx(OrderInterface $order, $stateTo, $stateFrom = null)
{
    foreach ($order->getInventoryUnits() as $unit) {
        if (null === $stateFrom || $stateFrom === $unit->getShippingState()) {
            $unit->setShippingState($stateTo);
        }
    }
}
```
```php
$this->get('sylius.xxx')->xxx($order, ShipmentItemInterface::STATE_SHIPPED, ShipmentItemInterface::STATE_READY);
```

@pjedrzejewski @winzou To what service this task naturally belongs to?