Going from Paris to Avignon in May but I like doing things at the last minute. I don't want to make a reservation on Ouigo, rather buy it when I'm ready to leave. Can I do this?
Nope you can only buy them in advance online. The TGV serves that route and will sell you a ticket last minute however
Not Ouigo but you can buy tickets at either a ticket window or ticket kiosks, at the station, for the next train.
Comments above are making the distinction (which I hope you recognize) between traditional, "full service" TGV Inoui trains and the more recently developed OUIGO budget brand of equally fast trains. The first is more expensive, especially to buy on short notice, but the second has more restrictions, including the required online purchase procedure (described at www.ouigo.com).
Last-minute travel decisions are very often costly these days. It's become a luxury to travel on a whim and discounts are rare. Trains and planes can control their load factors so that their prices only rise and a walk-up customer may pay double, or more, than someone who planned and purchased months ahead. Sorry.
When in May? It is a busy travel day due to the numerous holidays, and trains around those holidays will be fully booked. Some might be fully booked already! Frankly, I'd bite the bullet and book now. TGV Inoui Tickets are part-refundable anyway.