Late booking of train from Bordeaux to Paris, high prices, any other options?

Libourne is near Bordeaux and has TGV service to Paris. Your could drive to Angoulème and pickup the train, but generally, the price is the price.

are you sure that the Bordeaux car return is open on Sundays? Many in France are half day Saturday and closed on Sunday.

I’d just bite the bullet and take the train from Bordeaux. Hopefully, the OP is renting from Sixt which has a location at the train station in Bordeaux that is open on Sundays.

There is the bus. Flixbus we have used but not in France. It is much longer however, 7-10 hours. But the buses and newer and quite comfortable with toilet and wifi. Pricing depends on various factors but if purchased in advance looks like about $40 each for that Sunday.

Outside of major car rental locations in Bordeaux, a Sunday usually means "out of hours return". Possible in many places but always risky. If you do it, take plenty of pictures of the car when you return it.

With this is mind: have a look at returning the car in Brive or Limoges, and take the train to Paris from there. It's a bit slower, but the drive is shorter, and it is a cheaper route.

Here's why you have a problem: it's a Sunday, always a heavily traveled day. it's also the first weekend of the two-week fall school vacation and families have booked trains to Paris for months. I know grandparents on their way to Paris to babysit, and I know children being taken to stay with grandparents who live in Paris.
Whatever you do, stay off the road. The traffic jam as you approach Paris will be monstrous whether you are driving or sitting in a Flixbus. Take a train.

I see two solutions: stay on in the south and bite the bullet for the 22nd. Luckily, you see some available seats on this very heavy travel day. Or, you can turn your car in Saturday morning, somewhere open and take a Saturday train back to Paris. You'd have to put up with a day in Paris LOL. Bon voyage.

Did you look on the Ouigo website (not just the SNCF one)? Those are budget trains. They're old TGVs and run nearly as fast as regular TGVs, although are a bit more cramped. The one from Bordeaux to Paris runs direct to the Gare Montparnasse in the 14th. I've taken it many times.

When I looked for that day on Ouigo, I saw trains from Bordeaux to Paris at 65 Euros. Granted, other days it's just 25 Euros, but 65 Euros isn't bad for last minute on a big holiday week-end.

THANK YOU for all the replies, and because of many helpful answers here, and also the added info about this being a holiday weekend, etc,, it was helpful to decide fairly quickly to rule out the option of driving and just bite the bullet with the train from Bordeaux.

Thank you for those who pointed out the Sunday rental return and that the office is most likely closed Yes, I am aware the office is closed(it's Europcar) when I first rented it but was assured that there is an attendant in the parking lot when you return it to go over the car when we drop off, and yes, will take lots of photos and videos. Are there any specific areas apart from the exterior body, tires and rims I should also document? I rented through AutoEurope which we have always done in the past. My experience is they have a big enough clout with these companies that I prefer they deal with the car rental IF there are any issues. Thankfully, i have not had any problems so far to have to address with them.

Thank you for all the suggestions for alternative train stations, etc, and was able to quickly check those as options. In the end, the most practical and expedient option was to go with the original plan, and just bite the bullet as several pointed out. Fortunately, I read about the Carte Avantage and was able to get a discount, we were able to get a first class ticket on the InOUI since it was a small difference from the second class in the same train.

However, after being able to carefully go through the purchase which went through successfully on the SNCF website(and with a small glitch about my credit card first flagging it as FRAUD but was quickly resolved), the ticket showed seats that I did NOT choose. I double and triple checked my purchase before I hit VALIDATE and when the purchase was confirmed, it showed a different seat assigned, still First Class but on the lower deck and a seat number I did not choose. I will post about this question separately

It sounds as if when your first try didn't work, the computer defaulted your seat selection. I think I remember it happening to me. The good news is that you can see the countryside just as well. Usually, the bottom seats fill before the top in first where people have a seat choice.

Be sure to be there early. Some people with seats on the top like to drop their luggage off on the bottom, filling those racks first.

THANK YOU again everyone!