https://www.sbb.ch/en/leisure-holidays/trains-trips/rail-travel-specialtrains/panoramareisen/gotthard-panorama-express/purchasing.html

The link have all informations you need :)

16 CHF with a 1st class pass
36 CHF with a 2nd class pass

There are still a few local/regional trains that run on exactly same rails (its the old line before the new lower and much longer tunnel was opened a few yrs ago-even when the old tunnel was when it opened a major feat and made travel much faster as before) and thus without anything extra to pay.

You can also simply go to any swiss counter if you are in the swiss a few days before.

Thank you very much for the answers,

I have now managed to book the seats in the train. Still I don’t know how to book the boat from Luzern to Fluelen, with 50% interrail discount, and the first class supplement for the train. 

Any suggestions? We could buy the boot tickets only in the same morning in Switzerland, but I am afraid the boot will be full by than.

Thank you

As you cant reserve the boat you can still buy it at the ticket office when you are there. They have a own ticket office directly opposite of the station. Note usually the ship is quite full until Vitznau (here many change for the Rigi Mountainrail)