I traveled the nutshell route the June 27-28, 2014.
I purchased my tickets for the trains/ferry online using the above websites. You can print the tickets from your phone/tablet etc. For the bus, just pay the driver no advance purchase needed. For my trip the first bus was full, don't panic if this happens, there was another right behind it leaving at the same. As to train tickets, I believe you get a discount for advance purchases, they call it minipris pricing. I was able to get this rate when I booked a month in advance. You may have to call your credit card company to clear the purchase as it is a European web site, your card compnay may block the purchase as a security measure. I travelled from Bergen, stayed overnight in Flam(highly recommend) and then onward to Oslo. If you have some flexibility on the dates, you may want to verify if there will be a cruise ship in Flam the day you will be there, if so you should try to travel on a day when there won't be 2000 people disembarking for the day. On tip: don't rush from train to bus to ferry, you will have plenty of time to transfer, relax and enjoy the scenery.
If you have time in Flam:
Eat : Aegir Brewery
Snack: Gelato at train station
Activity: Kayaking-ok for kids-book ahead as it sells out(http://www.njord.as/)