/**
  * Upgrade a Order to a Invoice
  *
  * @param  integer $orderNumber
  * @return object
  */
 public function upgrade($orderNumber)
 {
     $handle = $this->getHandle($orderNumber);
     $id = $this->client->Order_UpgradeToInvoice(['orderHandle' => $handle])->Order_UpgradeToInvoiceResult;
     return $id;
 }