After exiting customs at CDG terminal 1, do we purchase a carnet of metro tickets AND an RER B bus pass to get to Invalides? It looks like we also need an RER C pass to get there... Thank you!
Paris Metro tickets are only good for subways within Paris city center, whereas the RER B from CDG Airport is far outside Paris city center. You'll need RER tickets to get into Paris city center, but these will also work for transfers onto the Metro (to get to Invalides for example) if it's one continuous uninterrupted trip.