Hello Beatrice2
Seat reservation in EuroCity is not obligatory for the route Thun - Domodossola. However, we recommend you to reserve a seat because this train is often very occupied.
Please indicate your GA as a discount card when your are ordering your ticket on sbb-tickets.cc/en to receive the corresponding reduction on the ticket price.
I wish you a great trip to Domodossola.
Regards, RahelW