protected function setShippingCharges()
    {
        $this->shippingCharges = 0;
        $orderAmount = $this->subtotal - $this->discount;

        if ($orderAmount > 0 && $orderAmount < config('cart_manager.shipping_charges_threshold')) {
            $shippingCharges = config('cart_manager.shipping_charges');

            if ($shippingCharges > 0) {
                $this->shippingCharges = $shippingCharges;
            }
        }
    }