Is it a good idea to purchase round trip tickets online? Will I have proof I can show on my phone? Or do I need to print them out? What is the best website to use? Any other tips? Is it easy to get a cab to the station? Thanks very much!
It’s a regional train so no need to buy in advance
They can’t sell out and price never changes
You can buy online at official Trenitalua site or use the app
https://www.trenitalia.com/content/tcom/en.html
As long as you can pull them up on phone no need to print
Taxis are readily available at the official taxi rank outside the station-if you are going to Bergamo Alta or there is a bus
ETA
Oh if you mean cab to station in Milan? Yes have your hotel call for one