public function returnItems($items, $discounts = null)
    {
        $data = array(
            'items' => array_map(function(ItemReturn $item) {
                return $item->serialize();
            }, $items)
        );

        if ($discounts !== null) {
            $data['discounts'] = array_map(function(ItemDiscountReturn $item) {
                return $item->serialize();
            }, $discounts);
        }

        $data = $this->twisto->requestJson('POST', 'invoice/' . urlencode($this->invoice_id) . '/return/', $data);
        $this->deserialize($data);
    }