Paris to Zürich / Zurich by train

SNCF is the national rail operator in France and not so long ago it recently simplified both the range of tickets and the term and conditions of using them.
Therefore the cheapest tickets also become more flexible too.

Other improvements included a simple integration of travelling with bicycles and rail pass users being able to book reservations for national journeys, without paying booking fees.

Though a legacy of when SNCF sold international tickets under the Rail Europe branding is that OuiSNCF is available in multiple translations with the offerings on the site then tailored to international visitors to France.
However, some of the site's features, including the integration of mobility assistance into the booking path, are only available on the French language version.

Something else to look out for is that simplest way of travelling with tickets is to use the SNCF 'My Travel' phone app, so it's therefore often a good idea to book on the OuiSNCF mobile service.

Happy Rail is a Netherlands rail ticket agency which sells tickets for both national and international journeys within a range of countries including Belgium France, Germany, Sweden, Switzerland.

HappyRail doesn't charge booking fees in the conventional sense, but if you're not Dutch you can expect to pay additional transaction fees, which will be added to the total cost.
That's because the most common form of online payment used in The Netherlands is 'iDEAL' and HappyRail doesn't charge a transaction fee for 'iDEAL' payments, but only Dutch residents can sign up to 'iDEAL'.
Other forms of payment include Visa debit cards will incur a fee.