In planning to do this, did you book your nutshell tour, and book/buy the express route to and from Balestrand separately? We plan to extend nutshell over 2 days to do this but weren't sure of the best way to secure tickets.
We bought the train route from Myrdal-Oslo separately online with NSB to get the cheaper price, and the Bergen-Voss train tickets at the Bergen TI, with the rest (bus and boats) through Fjord Tours. I don't think we saved that much. It would have been easier to book the entire package through them, which would add the Flam-Balestrand round trip boat to the Nutshell route.