I payed with a card and have the banking app and a sort of receipt which has an infraction number attached.
I’ll go to enquire today. Are there any larger stations on the metro near the Louvre?
OK at least its not a scam.
If you started at Gare de l'Est then the station you were fined at is on the way to where you were going, and the route you would take is change to RER C at Invalides. But to be honest it looks like you were using a single to make multiple journeys.
If you had already visited Pont de l'Alma then the fine is valid, if you had been travelling around other places then the fine is valid. The 2.5e ticket is for a single journey. I suspect from exchange you report its not a '6pm' restriction, its that you started your journey at 6pm, and this got lost in the translation.
Here is a snip from the RATP website, the bit you missed
Metro-metro lines, RER-RER lines, and metro-RER lines in Paris for 90 minutes from the first validation (without leaving the network);
As a fairly regular vistor to Paris to be honest the various Navigo offering are much less hassle. If you going to be in Paris for more than 2 days then assuming these three days or more dont cross a Sunday to Monday its 31.60e for an all zones pass for the week (with the Navigo 'week' being Monday to Sunday). That covers everywhere, all the week. Buses, Metro, Trams, RER, and 'main line' suburban trains within the Ile de France so you are covered for trips out to places like Versaille and Disneyland. For shorter duration visits the days passes are a good bet. You can also get them on your phone as long as your phone is fairly modern (not sure if they have sorted the iPhone issue yet) or buy a smart card and top it up either at a stations or using your phone.
We have Navigo Decouvert passes which you need for the weekly pass, they cost 5e I think, and we can top them both up using my phone as it has the required NFC hardware. So when we arrive in Paris we already have our tickets and just go straight to our metro or RER for onward travel. The Decouvert pass requires a photocard which is issued at a manned ticket window, its a one off task with an odd sized non passport sized photo, but all photo booths in Paris offer them. The Navigo Easy doesn't need the photo but cant be used for weekly or monthly passes (I think).