CODE    /**
     * Add tracking information for one or more PayPal transactions
     * param array $trackers  Array of tracker objects
     *
     * return array|false
     */
    public function addTracking(array $trackers)
    {
        if(empty($trackers)) {
            return false;
        }

        // PayPal accepts max 20 trackers per call
        $trackers = array_slice($trackers, 0, 20);

        $body = array(
            'trackers' => $trackers,
        );

        return $this->request(
            'POST',
            '/v1/shipping/trackers-batch',
            $body,
            $this->generateIdempotencyKey('tracking_'.md5(json_encode($trackers)))
        );
    }

    /**
     * Update an existing tracking (status, carrier, etc.)
     * Endpoint: PUT /v1/shipping/trackers/{transaction_id}-{tracking_number}
     *
     * param string $transactionId
     * param string $trackingNumber
     * param array  $data (status, carrier, carrier_name_other, notify_buyer...)
     *
     * return array|false
     */
    public function updateTracking($transactionId, $trackingNumber, array $data)
    {
        if(empty($transactionId) || empty($trackingNumber)) {
            return false;
        }

        $body = array_merge(array(
            'transaction_id'  => $transactionId,
            'tracking_number' => $trackingNumber,
        ), $data);

        $trackerId = urlencode($transactionId).'-'.urlencode($trackingNumber);

        return $this->request(
            'PUT',
            '/v1/shipping/trackers/'.$trackerId,
            $body,
            $this->generateIdempotencyKey('update_tracking_'.$trackerId.'_'.md5(json_encode($data)))
        );
    }