If you’re only doing a round trip Basel <> Paris, you don’t want a pass. Just buy regular tickets. Tickets start from €39 each way, 

There’s currently a sale on, so depending on your dates, you might be able to get a bargain.

Thank you ​@ralderton for your answer. Forgive my ignorance, but where would I purchase the tickets in advance? On the Eurail site, it says I have to purchase a pass first. Is there a different place or site to go to?

My day trip is really way in advance - June 20th, 2025.

Use the companies websites. SNCF in this case.

Right now only expensive direct trains are shown -> I would expect more trains/cheaper fares to appear in January-February 2025. Just be a bit more patient. :)

Tickets are sold through many retailers. As ​@thibcabe says, you can buy them from SNCF, for example.

For a quick and easy check, have a look at Rail Europe. (Not necessarily where I’d buy the tickets from, because they charge booking fees, but a really easy way to check prices.) You can do a dummy booking for closer dates, to see what’s available.

I picked, at random 10 January for Basel-Paris, and it shows the direct and indirect trains - including a fairly early direct departure for €39,