code: public static Batch addShipments(String id, String[] shipmentIds)
  			throws AuthenticationException, InvalidRequestException, APIConnectionException, APIException {
  		BatchShipmentId[] array = new BatchShipmentId[shipmentIds.length];
  		for (int i = 0; i < array.length; i++) {
  			array[i] = new BatchShipmentId(shipmentIds[i]);
  		}
  		Map<String, Object> params = new HashMap<String, Object>();
  		params.put("__list", array);
  		return request(RequestMethod.POST, instanceURL(Batch.class, id) + "/add_shipments", params, Batch.class, null);
  	}
docs: Add shipments to an existing batch provided by id. This takes shipment
  IDs which means shipments should have been already been created using
  {@link Shipment#create}. This method corresponds to
  https://api.goshippo.com/batches/<i>BATCH OBJECT ID</i>/add_shipments endpoint
  defined in https://goshippo.com/docs/reference#batches-add-shipments
  
  @param id
  Batch object ID
  @param shipmentIds
  Array of shipment Ids to be added to the batch
  @return The Batch object after shipments have been added