Paris to La Rochelle 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.

And it also recently launched SNCF Connect as replacement for its Oui.SNCF booking service.
You can set up a low price alert to let you know when the cheapest price is available for a future journey.

Something else to look out for is that simplest way of travelling with tickets is to use the SNCF Connect App, as booked tickets can therefore be stored on your mobile device, which can then be shown to the train conductors as proof of purchase.

SNCF Connect on Google Play

SNCF Connect in the App (Apple) store

SNCF also operates RailEurope, which is in effect a dedicated online booking service for making bookings from outside of Europe.
If you reside outside of Europe you may discover that SNCF Connect rejects phone numbers or card numbers, but RailEurope won't do so.
Though RailEurope will add booking fees when making a purchase

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.