The bus can be booked at a ticket office in Switzerland or Italy.

There are other routes, but they aren't shorter.

thanks for your quick reply. We have 1st class eurail passes. Would they cover the bus? I would like to reserve seats in advance for the bus for peace of mind. Which site is best to reserve the seats?

Yes your pass is valid. 1st/2nd class doesn't matter.

As rvdborgt said the bus cannot be booked online. Book at any counter in Switzerland (3 CHF passholder fare).

Or you buy a 13€ ticket online in advance... but availability won't be an issue on an evening so personally I'd rather wait. 

Hi, is a 13 min exchange at Spiez enough time? So sounds like if I reserve seats in advance for the bus portion of my route it will be 13euro vs 3CHF if i book once in switzerland? Are there any othe routes you would suggested from Interlake to Milan? We would like minimal changes.

13 min is enough. Yes 13€ for an online ticket.

You could take the long way round via Lucerne and Lugano. 3 trains and fully reservation-free. 1h longer than usual due to the Gotthard Base Tunnel closure. For example:

- IR Interlaken Ost - Lucerne 16:15 - 18:04

- IC21 Lucerne - Lugano 18:18 - 20:48

- RE Lugano - Milano Centrale 21:02 - 22:17

So this is the journey I am planning to take on Aug 6th (see below). Are these trains usually on time? Given it is late at night, I am worried that if we miss any connection due to delays, we may not make it to Milan that night. Would the longer route through Lucerne and Lugano be a safer bet?

Also looks like seat reservations can only be done at the station for the 2nd and 3rd leg. Any risk that the trains might get full?

6:30pm-6:52pm Interlaken to Spiez

7:05pm-8:12pm Spiez to Domodossola

8:30pm-9:50pm Domodossola to Milan (bus)

Hi, below is the schedule I am seeing for Interlaken to Milan on Aug 6th. I was wondering if these trains are usally on time? I am worried about the connections and getting stuck somewhere since our route is in the evening? Also, looks like I can’t reserve any seats in advance anymore for the bus- hopefully that won’t be an issue?

These trains are on time yes and the bus will wait for the EC no matter what.

No reservations until Domodossola.

Then as I said either use https://www.trenitalia.com/ for a 13€ ticket or get a passholder reservation (3 CHF) at a Swiss station.

The evening departure (2 coaches) is unlikely to sell out. When I was in Domo earlier in June they were barely half full.