Swiss Rail pass purchase--here or in Switzerland?

Same price whether you buy online from the SBB website, or at a border or airport station.

You save a few minutes at the station if you buy online before leaving home. If you are crossing the border by train, buying in advance saves getting off the train, and back on a later one. A Swiss pass is of course only valid from the border, you need to buy "foreign" tickets for the train up to the border.

Website to buy passes: https://www.sbb.ch/en/leisure-holidays/travel-in-switzerland/international-guests/swiss-travel-pass.html
Makes sure the card you use on this website is authorised for use in Switzerland, which is where the transaction will be booked.If you buy on the SBB website, you get a "print-at-home" ticket.

it is very easy to buy your train ticket/pass at the Airport. Just go to the window and talk to a real person and complete the transactions using your CC. Its easy - opposite Starbucks at Zurich airport. No lines when we were there in July. Then go to the Coop nearby and get some snacks for the train. There are several ATM machines close by for your Euros. If you get a pass they will provide you with a plastic protective pocket. A pass is only valid with an ID so make sure you have your passport with you when you are on the trains.
Margaret

Can a Swiss Travel Pass be purchased at an Italian train station (ie Domodossola) or Milan where travel from Domo is part of the pass, but travel from Milan to Domo on the same train would not be, and require a trenitalia ticket? Or only at an SBB ticket window? Or on line?

To answer Karen's question:
A Swiss Pass can only be purchased online or at any manned Swiss-owned station, which, AFAIK includes Domodossola as it is the border station (joint Swiss/Italian), but NOT at Milan. No SBB ticket office in Milan.

"Milan where travel from Domo is part of the pass, but travel from Milan to Domo on the same train would not be, and require a trenitalia ticket?" - A Swiss pass is only valid on Swiss rail lines. In this case from Domodossola onwards.
See official map of where it is valid here: https://www.sbb.ch/content/dam/sbb/de/infotexte/uebersichtskarte-sts.pdf

As you say, if you have a Swiss pass, you need a separate Italian ticket for Milan to Domodossola. The advantage of getting your pass in advance (online) is you don't need to get off the train at Domodossola.

@mpaulyn - "There are several ATM machines close by for your Euros." -except in Switzerland they use Franks!

We bought ours online a couple weeks in advance. No advantage, we just knew we’d be tired, traveling with kids, and it’d be one less thing to do once we got there.