The worst trains for reservation prices are Thalys and international TGV services. Avoid these and the most you should be paying is €10 or possibly €20 on internal French TGV.

Your first choice is probably a more interesting route, just use local trains from Switzerland to France then look for connections towards Hendaye.

DB cannot make passholder reservations for French trains, use https://travel.b-europe.com/eurail-ge/en/booking-tgv?#TravelWish for internal TGV reservations

along with pass cover number

Basque? Seeing the name.

It is not clear if you want the scenic route with multiple stops or rather fast. In any case München is via a more roundabout route.

As above: use INland TGV-if booked longer before on not-so-wanted lines its mostly 10€/seat. There are also such bypassing Paris till Bordeaux-and onward, from Strasbourg. However (with the same situation the other way last oct.) I found that SNCF may simply bar passes from them or price them far higher as TGV to+fro Paris.

You have to aim for Hendaye/a, then use the BASQ! Euskadi trenlerinde or so-or the bus, via Donostia to Bilbao-these are not in passes, but quite cheap-though I think you must also buy a chipcard if not having already.

Thanks for the information AI_G. I will look again the route via Lyon, but my impression was that it was difficult to avoid TGVs between Lyon and Bordeaux and that rail tickets there were reeeeeally expensive. The site you mention to make researvations for French trains only works with Interrail pass, and I was hoping to get an idea of the real price after adding seat reservations before going for it Good to know the price range EUR10-20 on TGV. Thanks again :)

Thanks for the information, mcadv. yes, basque. Munich has sentimental value. I am a bit flexibel with the route, but since I have never done such a long rail journey, a bit cautious too. I just hope to avoid getting stranded somewhere in winter I guess I am too late with planning. Prices southern France are incredible high. 

AFAIK, there are no regular domestic TGV services in which passes are not valid.

So please specify in which trains, in your opinion, SNCF barred the use of passes and how you found out. As always, if you can't provide any more specific information, then the community should ignore your text.

Possibly referring to Ouigo services, which are not valid on interrail. 

For prices for SNCF domestic check out their own website which shows bouh Inoui TGV (included with IR at 10 or 20 euro) and Ouigo (No IR).

,https://www.sncf-connect.com/en-en/

AS an alternative from Munich consider a routing through the alps to Italy and then along the Cote d’azur to Marseilles. 

But that's not a regular TGV service.

Thanks for the info regarding Ouigo, Yorkie! I will look into the other route, but I think will be too long, particularly when heading for north Spain. Thanks for sharing valuable info and suggestions :)