Avoiding the A13 Free-Flow Motorway

You can put your destination into Google maps and choose "avoid toll roads" - . Not sure about bridges but all tolls we encountered were able to be paid with cards. We were in a different area of France but hopefully it's the same.

If you really want to avoid the A13 you can take more north the A15 to Rouen. For this you have to follow the A86 clockwise from Antony to the exit to Cergy-Pontoise. More west the A15 is named D14 and a freeway till Magny-en-Vexin and continues further as national road D6014 to Rouen. This is actually the old road between Paris and Rouen. Followed this road a few times and is a bit boring to my idea.

Be aware that the A86 is partly a toll tunnel north of Versailles. And ofcourse you have to deal with the (slow moving) traffic through the outskirts of Paris.

On the way from Les Andelys to Forges-les-Eaux is Lyons-la-Forêt not to miss.

From the A29 you can cross the Seine at the toll free Pont de Brotonne and take from Bourg-Achard the national road D675 running mostly parallel to the A13 all the way to Caen. But with all the speed limits it’s a very time consuming drive, a few breaks will make sense.

I have just looked into the freeflow payment system for use in July, and it's very straightfoward. You can even register your email address to get a reminder before the deadline.

The official website in English.
https://www.sanef.com/index-en.html

Couldn't (or would'nt) figure out the Free-flow while in France last 2 weeks. Actually sat and figured it out after I returned home and paid tolls online. Easy-peasy, better than fumbling with credit card at toll booths.

The flux libre system is very simple and easy to use. There's no reason to go out of your way to avoid it.

But if you still want to inconvenience yourself, there are lots of "N" roads and even slower "D" roads that you could take. It shouldn't be hard to plot a route by using something like Via Michelin and directing it to avoid toll roads.

I have registered with Sanef, but since I will have a rental car I'm guessing that I will not be able to pay for tolls until I can plug in the license plate number of the vehicle. Is that correct? Or will the rental car company, in this case Avis, already have a device installed on the car which they will use to bill me for tolls (and possibly add on other fees)?

Registering the vehicle is optional.
The number plate will be recorded using ANPR.
Once you have done the trip, within 72 hours you can just go onto sanef.com, enter the reg. no. and it will tell you how much to pay.

Apparently the 72 hour guideline is not fixed either. It was over a week before I got home and noodled it out. No penalties assessed.