After a week in Paris for the Rick Steves' tour, we plan to take the train to Amsterdam. We will be leaving Paris on Sunday, April 14th. Should we purchase those tickets online now or would it be better to buy them in person at the station on the day we plan to leave?
You can purchase them at the station. It will however be a lot cheaper to buy them online now.
Tickets will be for a specific train, once you buy tickets you cannot change your mind and travel on a different train.
The train company is Thalys: https://www.thalys.com/fr/en
For 14th April you are already too late to get the absolute cheapest tickets. Prices are currently in the range €121 to €135
You will get a "self-print" ticket e-mailed to you.
If you get them 3 mos or so in advance they are about 35 Euro; we have done this many times. Alas, you are probably too late to get good fares but they only go up till the date of travel. It costs 3 or 4 times as much to buy long haul trains at the last minute as it does if you get them months out.