I just don't want them to sell out, but I don't want to miss it if our flight is delayed
You can see the price difference between flexible and non-flex tickets by checking online at https://www.oui.sncf/
It's unlikely the train will sell out unless it's during an especially busy time (just before Christmas or national holiday).
You also have the option of purchasing tickets that are a fair amount later than your flight arrival and changing your ticket to catch an earlier train with a small fee. To get an idea of fees:
Superflex and PRO tickets would give you the most flexibility and lowest cost of ticket changes. Loisir tickets also allow changes with a 15€ fee per ticket.