The Eurail website is not the most reliable website. It often struggles when there are multiple trains to book and works better if you book one train at a time. In this case, you only need to book one train in total (Paris - Mulhouse for 10€ per person). Mulhouse - Basel can't be booked and Basel - Spiez is not necessary (reservations in Switzerland are a waste of money IMO).

The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website.  You can look at the guide in the link:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

For this journey in particular, you can use this B-Europe website. You need a pass cover number, which you can generate here if you have a mobile pass.

These are USD prices-noone here uses that money.

Its likely that you booked/tried to a through train TGV into the Swiss-this cost an awful lot more as you noted-hence rian advice to break in Mulhouse.

We did suffer a systems error on our reservation system in beginning July which might have caused this issue. The price of this journey depends on which route you take. The cheapest seems to be to travel from Paris Est → Strasbourg → Basel  SBB → Spiez. The only reservation required here is for the first leg from Paris Est → Strasbourg for €12 per traveler. I hope that you managed this reservation. In the event of technical glitches you can always check our alternative reservation methods