Tickets to Italy are only available in the webshop, using www.sbb.ch, yet not in the SBB Mobile app.
The Half Fare Card is valid as far as Domodossola. When you select half fare, the system will automatically alter the fare for the Swiss stretch.
You can buy it with Trenitalia, too, no difference. If you travel on the Eurocity, you need a reservation from Domodossola to Milan.