public function getOrderMeta($property)
    {
    	/** @var \WC_Order $order */
	    $order = $this->invoiceSource->getOrder()->getSource();
	    $value  = get_post_meta( $order->id, $property, true);
        // get_post_meta() can return false or ''.
        if (empty($value)) {
            // Not found: indicate so by returning null.
            $value = null;
        }
        return $value;
    }