I didn't have time to register for a tag, but if anybody's interested I had an experience trying to pay (cash or card). On the A29 going north from Honfleur, got the ticket OK at the first booth but when I tried to pay when leaving, the machine wouldn't take it. There was a voice instruction in French, too quick to follow, but that wasn't the problem, I knew what to do, but it wouldn't work. There were also help buttons for French, English or German but no answer when I tried that. I reversed and tried several booths but no joy. A couple of guys tried to help, but they couldn't understand what was wrong. Some other drivers were also having a problem, but they all got through eventually. Needless to say, there were no human operatives available.
In the end I waited near one of the tag lanes, and when an HGV came I tailgated him and got through! Reasoned that the machine wouldn't distinguish my car from a trailer he might have been pulling, and it worked. A somewhat stressful half-hour.