Hello Shiequuequue58
We do not offer support in Spanish, but in one of the Swiss national languages or English.
The most convenient is an Eurocity (EC) ticket from Chiasso to Milan. Your Swiss Travel Pass is valid to the border station, Chiasso. From there, you will need a ticket. You can get it at sbb-tickets.cc/en or trenitalia.com. With this ticket, you could take a direct EC train service from Zurich HB to Milano Centrale.
→ Tip: You can book the ticket from Zurich HB to Milan, and choose “GA travel card” as reduction type. By doing so, you will not pay for the Swiss part (which is covered by the 15 days STP), but for the Italian stretch; however, the advantage of doing so, you will receive a seat reservation for the entire journey.
Note: this works for holders of the Swiss Travel Pass or GA Travel Card only; with a Saver Day Passes and other day passes, a booking Zurich HB to Milan stating the GA Travel Card is not possible / not allowed.
Furthermore, there is also an Arcobaleno ticket available. This ticket is very inexpensive, but you would be required to change trains in Lugano and continue with the regional express train (RE) to Milan.
Kind regards,
Roland