public function buy($params = null)
{
$requestor = new Requestor($this->_apiKey);
$url = $this->instanceUrl() . '/buy';
if ($params instanceof Rate) {
$clone = $params;
unset($params);
$params['carrier'] = $clone->carrier;
$params['service'] = $clone->service;
}
list($response, $apiKey) = $requestor->request('post', $url, $params);
$this->refreshFrom($response, $apiKey, false);
return $this;
}