There are engineering works on the Simplon route (Domodossola - Milan) over the summer. Buses replace trains on that leg. In addition there are 3 daily buses from Geneva/Lausanne to Milan.

Passholder reservations are available at Swiss ticket counters. I’ve just tried the one in Geneva last week and they cost 13€ for the direct bus (3€ for Domo - Milan).

In addition they seem to be on https://www.raileurope.com/en (desktop version, click on add railpass). Feedback appreciated.

There are very frequent trains between Geneva and Lausanne. Have some margin before the bus. The same one also leaves Geneva main station at 09:15 if you prefer.

@thibcabe Thank you sooo much for your response.  It makes sense now.  Would you know if the passholder reservations are available online/ or in Paris (where I’ll be prior to going to Geneva) or only at the swiss ticket counters?  I don’t seem to be able to buy the bus passes at raileurope.com unless I’m doing something wrong.

Passholder reservations are only possible on the desktop version of raileurope.com the mobile version does not have the option to add rail pass.

Although the direct bus from Lausanne is the quickest option it is a long 4 hour trip on a bus, there are other options available that would be train throughout by going via Zurich.

@Al_G Thank you for your response.  I agree that 4 hrs is long on a bus but it is shorter than the train. On the other hand the train is more scenic and probably less of a hassle.  I’ll check out the trains.  

I just did a test on the Rail Europe website but I can't book a pass reservation for the bus there. Did you rally see a price there?

Geneva/Lausanne - Milan works (run partially by SBB so I guess that helps). Domodossola - Milan doesn’t (entirely Trenitalia). I should’ve been clearer. :)