If I buy TER train ticket online can it be used for a later train if I'm late?

There are several price categories on most French trains, all with refund/reissue restrictions. While a full-fare ticket may allow a small window for re-booking, such as one hour after the departure of the train, others don't. Read all the rules on screen before buying.

For instance, these are the TER pick-up rules I see on a test booking at Voyages-SNCF.com:
You retrieve your ticket, in France or at the Luxembourg station at a Self-Service terminal, when you like and up the last minute before departure. Virtual cards and bank cards without Chip and Pin cannot be used for withdrawals at SNCF-ticket machines. The bank card used for payment and PIN number will be requested when withdrawing your tickets.

TER tickets are not reserved but do have an expiration date; it should be long enough to be flexible for your flight arrival. I would not usually pre-buy for connection with an incoming flight due to reservation issues and probably wouldn't do it this time, either, since it sounds like a hassle.

I can see no reason to purchase them in advance. According to the SNCF website http://special-offers.en.voyages-sncf.com/en/France-train-TER

Fixed fares: prices stay the same, even at the last minute!

In other words, the price difference depends on how much flexibility you want your ticket to have. If you're standing in a train station and planning on taking the next train out, you don't need flexibility and won't pay any more for the cheapest fare than if you had bought them months in advance.

The only reason to purchase TER tickets in advance is avoiding the line to buy on the date of travel but you still have to wait in line to pick up the ticket because I am pretty sure that you cannot print TER tickets at home so no real benefit for a person flying into Paris to get a TER train. It is possible to have them mailed to you but I don't know anyone who has done that. For TER tickets, you can take the same type of train, traveling the same route, at any time departing within 7 days of your reserved date. Please be mindful that certain fare conditions do restrict you from taking peak hour trains.

SNCF does not mail to the USA. I suspect (without testing it) that agents like Rail Europe and Capitaine Train will also be selling tickets that require pick up in the station.

For a regional train I like to buy them a day or two ahead so I don't miss the train trying to buy them the day of. That happened to us when traveling to Vernon -- the line for tickets was over an hour and we missed our train and went the next day. Buying early doesn't give a price break like it does for long haul trains like the Eurostar, Thalys or TGV.

Go to seat61. com. He is the boss on rail info in Europe.