Is the Swiss travel pass site a safe and reliable way to buy our half fare cards and Bernese Oberland train passes?
Can you provide a link to the site you are referring to.
Is it the SBB site? If so, that is safe.
This is where I buy my Half Fare Cards https://www.sbb.ch/en/leisure-holidays/inspiration/international-guests.html. If you have the SBB app, your HFC will appear on it.
The Berner Oberland Pass can be purchased here https://www.bls.ch/en/fahren/fahrausweise/abonnemente/berner-oberland-pass. You can save the passes as PDFs on your phone to show the when you take a lift, but, train, or boat.
Of course, you can buy them on arrival too.