You don’t need any reservations in Switzerland, and you don’t need any reservations on TERs in France.

So you can go by TER, for example, Paris-Mulhouse-Basel, then onwards. It will take longer, but avoids any reservation fees. It will probably leave from Paris Gare de l’Est, which is a 10 minute walk from Gare du Nord.

You can find these in the Eurail planner by choosing Filter > No Seat Reservations.

The fastest journey I see that requires no reservation fees would be Paris-Belfort-Meroux-Glovelier-Le Crux des Biches, taking 7h48.

You could save yourself a couple of hours by instead going Paris-Mulhouse or Paris-Strasbourg by TGV and paying a €10 or 20 reservation fee, then continuing on unreserved trains after that.

Which service has the most cheaper seat reservation seat? Where can I buy it?

Meroux is a TGV station actually (called Belfort-Montbéliard TGV) so you can easily connect there with a TGV from Paris Gare de Lyon.

But the true reservation-free way is via Belfort as agreed. :)

A trip on the TGV completely within France will cost either €10 or €20 (limited availability on all routes at €10, then the rest of the seats are €20).

If you take it across the border to Basel , it will cost €29. 

You can book both via the Eurail reservations service, they charge an extra €2 per reservation. https://www.interrail.eu/en/book-reservations#/

The regional and TER trains don’t requite reservation.

Do you have a specific date in mind ? Itinerary examples :

- TGV Paris Gare de Lyon - Belfort-Montbéliard TGV 07:22 - 09:37 10-20€

- same station : climb the stairs or take the lift

- RE Meroux TGV - Glovelier 10:03 - 10:55

- R Glovelier - Le Creux-des-Biches 11:41 - 12:30

Or fully reservation-free :

- TER Paris-Est - Belfort 08:42 - 12:46

- TER Belfort - Meroux TGV 13:48 - 13:56 (the same train leaves at 12:48 so you could make the earlier connection but don't count on it)

- RE Meroux TGV - Glovelier 14:03 - 14:55

- R Glovelier - Le Creux-des-Biches 15:41 - 16:30