docs: public void purchaseReservedVolume(String volumeId, int reservationLength, String reservationTimeUnit) {
          this.purchaseReservedVolume(new PurchaseReservedVolumeRequest()
                  .withVolumeId(volumeId)
                  .withBilling(new Billing().withReservation(new Reservation()
                          .withReservationLength(reservationLength)
                          .withReservationTimeUnit(reservationTimeUnit))));
      }