Soahauvu96
The amount of the 20.- fee rather sounds like the ticket. If it was a reservation, you should see some coach and seat number on whatever receipt you got.
As Danhiel already stated above, make sure that you also have some valid 2nd class ticket along with that 1st class upgrade ticket (which you had with the STP, but NOT with the half fare pass)
If in doubt regarding reservations, contact the Golden Pass operator MOB railways, as it is them managing the seat reservations on that line.
