Hello rdlp916
You can buy two separate tickets:
— St. Moritz - Chiavenna for the bus
— Chiavenna - Venezia Mestre for the train
Note that you have a second problem if you want to buy first class tickets, because you have only 2nd class between Chiavenna and Colico. In this case, three tickets were necessary:
— St. Moritz - Chiavenna for the bus
— Chiavenna - Colico for the train 2nd class
— Colico - Venezia Mestre for the train 1st class
I suggest to buy the tickets for Italy on trenitalia.com or in the Trenitalia app with an account. You have five advantages:
— you can make the mandatory check-in for regional trains in Italy in the Trenitalia app
— you can choose your seat in the trains with mandatory reservations
— you can change the time and date of the departure online or in in the app (if the pricing policy allows this change)
— you can cancel the tickets online or in the app (if the pricing policy allows cancellations and refunds)
— you can obtain a compensation if the train is late in the app (not always possible)
Kind Regards
Danhiel