public function calculate($aShippableItems, $oBasket)
    {
        $iCostPerItem = (int) $this->getSetting('iCostPerItem') ?: 0;
        $iItemCount   = 0;
        foreach ($aShippableItems as $oItem) {
            $iItemCount += $oItem->quantity;
        }
        return $iItemCount * $iCostPerItem;
    }