Train from Paris to Reims and Back

If you think that a different site will be cheaper, sorry, in general that will not be
the case.

thetrainline.com currently has a discount code "goeuro10" for 10% off the booking.
But I am not quite sure what prices you're seeing. I see a $50 RT on 10/28, which
isn't particularly outrageous.

The two sites I recommend are the the Trainline site mentioned above and the SNCF site that you have already found. I know of no reputable site where you can buy cheaper tickets on French trains. Reims is about 90 miles from Paris and you can get there in less than an hour by train. With that in mind, the prices may not seem quite as high. In any event, you may want to purchase your tickets sooner rather than later as the lower priced tickets may be gone closer to your travel date.

Thank you both! And that code will be helpful! So it seems like I should definitely book round trip tickets instead of buying a return when in Reims?

Tickets only go up in price, so buying your return ticket when you are in Reims means it will be as expensive as possible. Of course, the disadvantage of buying in advance is that it locks you into a specific time.

buying tickets last minute is both the most expensive way to do it and also you run the risk of the train being sold out. The best train is just 45 minutes from Paris -- TGV. If you book early you can get pretty cheap tickets. We lucked out when we went and were able to snag cheap tickets last minute mid week -- but that was luck -- usually you get the good prices months out.

We had hoped to get tickets from Carcassonne to Montpellier last minute last year as we wanted to not be bound by a specific time. They were all sold out on the TGV and we ended up scrambling for a seat on the local train that doesn't have seat reservations. It was packed to the gills. I am pretty good at snagging seats under those conditions and we did get seats, but lots of people were standing.

Train tickets in Europe are not really round trip. You can buy them at the same time but they are two one way tickets; you do not get a price break buying round trip tickets.

Italy does have same day return tickets with a discount on Frecce trains, but again, must be bought well in advance for a good discount and are nonrefundable but modifiable.
https://www.trenitalia.com/en/offers/same-day_return_offer.html

thanks Sam. did not know that

A related question - if we get a train that arrives to Reims at 9:15, is 9:50 cutting it too close for a tasting at Ruinart?

Cutting it close but you'd want to take a taxi from the station to there, walking or the trolley won't be enough time.