May 1, 2013 - 8:59 AM
I'm not sure why you mention Bern. If Thun is your destination, then book tickets from Italy to Thun, and from Thun to Paris.
You don't need the SBB site to book these tickets. Did you try the link I gave you in my last post? That's where you should be able to book all your tickets: Venice to Milan, another one from Milan to Thun, and another one from Thun to Paris.
I hope this works out for you!