Hello Liza
Thank you for your request.
There is no travel card, that would cover the whole journey up until Milan. All travel cards are valid to Domodossola. In this case, I’d suggest to buy two different tickets. One for the travel from Zürich to Zermatt and then a separate one from Zermatt to Milan.
Tickets can be bought on sbb.ch, SBB Mobile or even at the SBB travel centers and SBB Contact Center with the number 0848 44 66 88 (or +41 51 225 78 56, if you’re using a non-swiss phone connection). Please be advised, that the SBB Contact Center only sells international travel ticket by phone.
I wish you a pleasant day.
Regards,
SimoneMB