I am landing late evening at ORLY. I plan to stay overnight at Antony and then travel by train in the morning to CDG airport for our flight the next day. I understand the fare for this (ORLY to CDG) is 20 euros and includes the ORLY to Antony Shuttle. Can I buy the ticket at night and use it to get to Antony that night and then use the ticket the next day to get to CDG? Is it valid the next day?
A Paris single-use train ticket is only good for a period of time after it's used in a turnstile / gate to enter a platform or fare-paid area. It's about 2 hours for airport tickets. I'd recommend only getting separate Orly - Antony ticket and another Antony - CDG Airport ticket. You can purchase them both at the same time and use them when needed. The tickets themselves don't begin their expiration countdown until they're used in a fare gate / turnstile.