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@Ianinam just said as I was composing my response, there's no "direct" way to purchase a train ticket in advance from CDG to SJPP - you need to get from the airport to Paris Montparnasse first and then catch the train to SJPP from there.*
(*Edit: I stand corrected on the above - there are trains from CDG to Bayonne via Bordeaux and Dax that don't require going through Paris Montparnasse, but the schedule is still not amenable to getting to SJPP before mid-evening unless you can leave CDG by early morning - see train schedules in my post below.)
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It takes about an hour to get from CDG to Montparnasse (via RER B, changing to the Paris Metro at Saint Michel or Denfert-Rochereau). There's an RER B train from the airport to central Paris about every 10-15 minutes during the day (no need to purchase this part in advance - I'm not even sure you can). I'd give yourself at least a half hour after you get your luggage and go through customs to grab some coffee and get to the RER station, so we're talking at least 90 minutes post-customs to get to Montparnasse after your plane lands.
The RER B line is a direct rail link between Paris (Denfert-Rochereau, Saint-Michel-Notre-Dame, Châtelet-Les-Halles, Gare du Nord) and Paris-Charles de Gaulle Airport. Times, journey, schedules.
www.parisaeroport.fr
You can buy tickets from Paris to SJPP (via Bayonne) in advance via Trainline or RailEurope - or via the SNCF (French railway) website or app, which may save you a few Euros.
Trainline, your quick & easy way to book train tickets across Europe. ✓ Save 61% on UK Rail ✓ Live Train Times ✓ Save with a Railcard ✓ Get the app
www.thetrainline.com
There are about half a dozen trains a day from Paris to Bayonne, where you'll change to a train –
and then a bus, depending when you're going – to get to SJPP. (The whole Paris to SJPP journey can be booked at the same time though - you don't need to book each of those legs separately.)
The trip from Paris Montparnasse to SJPP (via Bayonne) is about 4 1/2 hours.
So if you're landing at CDG mid-morning you probably won't be able to catch any trains to Bayonne/SJPP until 2 PM or later, which will get you to SJPP by 6:30 PM at the earliest. Needless to say, it will be a very long and hectic day of travel, especially after an overnight flight from the U.S.!
Have you considered spending a day/night in Paris to rest up and adjust to the time change before heading to SJPP? You can catch a 10 AM train from Montparnasse the next morning, and be in SJPP by 4 PM or so.