Hello Keawoungoo15
Probably you have noticed that you will travel with Trenitalia Bus services from Domodossla to Milano. With your STP you can travel from Zermatt to Domodossola. May be it is easier to ask tickets only for Bus services from Domodosola to Milano Centrale, for 13 Euro.
MGB Travelcenter in Zematt can also assist you for tickets.
Kind regards
Christoph