Milan train to Tirano & Bernina Express to Zurich

We are excited about our upcoming trip to Italy, including a stay in Milan. We plan to travel by train to Tirano on Sunday, Sept. 16. We will be staying in Tirano that night and then traveling to Zurich by way of the Bernina Express on Monday, Sept. 17. I have read and seen on the internet that the price is better if one buys a ticket from Tirano, Italy to Singen, Germany and simply gets off the train in Zurich. Is there any advantage in buying tickets for either the Milan to Tirano or the Tirano to Zurich (Singen) now? Does the price go up as it gets closer to the trip date. Will the price be the same if I purchase at the train station in Italy? Any help on either of these train trips will be greatly appreciated.

Thank you,

Bill