/**
* @param string $shipmentExternalId
* @param string $rateExternalId
* @return ShipmentTracker
*/
public function buy($shipmentExternalId, $rateExternalId)
{
$shipment = new EasyPost\Shipment($shipmentExternalId);
$shipment->buy(['rate' => ['id' => $rateExternalId]]);
return $this->getShipmentTrackerFromEasyPostShipment($shipment);
}
