Unexpected €55 Europcar charge 1.5 months after returning rental car in Avignon

I'd bet it's toll related. Did you blow through the automatic payment booths at the péage rather than using tap to pay or cash?

we tapped our CC on the machine when we pass the toll

It could be the fee an agency charges when requested to give driver information to the authorities when a traffic violation occurs. You are absolutely correct that calling them is the best approach to solve this mystery. We can only make guesses.

My first guess is the hated, administrative fee for giving your id info to the local authorities for a traffic violation. -- could be parking, no travel zone, speed enforcement, etc. If it is the administrative, you can expect a traffic violation notice in the next six months or so. Good luck.

jathaca,
Back in 2017 a few months after our return home from France, we got a charge from the rental company (I can't remember which one) for about $30-$40. That was their fee for providing our info to the French authorities so they could send us a ticket for speeding on the autoroute. (It was deserved and was a camera which caught us.) A couple of weeks later the ticket arrived (by email, I think, but could have been snail mail, though not likely). We paid he ticket right away, as the fine goes up if you delay in paying. That was explained with the ticket. The rental company fee isn't a scam, but it is overpriced. The car is registered to the rental company and the authorities do not know who the driver is, hence the notification by the rental company.
I am guessing you will get another surprise from the French authorities if it was a moving violation. If it was a toll or parking fine, I can't say whether the 55 euro is the only thing you will get as we never had a problem with those things. I would contact Eurocar for a more detailed explanation of "Europcar AVNL03 Avignon".

€55 is exactly the fee Europcar charges for passing on your details to the authorities in the event of a fine (speeding, parking, etc.). Note that Hertz, for its part, charges €20.

So, as Judy said, expect another "surprise" in the coming days.