FYI, the Eurail website can't handle TGV 20 euro seat reservations (there apparently are very few 10 euro TGV seat reservations that is can handle, and these sell out quickly). When trying to purchase from shopping cart, during the verification step, the following error occurs:
"One of the legs of your journey could not be booked
One of the legs of your journey is not available anymore. Please try searching for another connection"
The work around is to purchase the seat reservations from https://travel.b-europe.com/, BUT you need a eurail "pass cover number" to do this. This "pass cover number" is obtained from https://www.eurail.com/en/book-reservations#/generatePassCoverNumber.
This has really been painful, but our 10 trip second class eurail pass will have more than paid for itself after our first four trips (London to France to Germany over 2 weeks).