Autoeurope is a good rental option. There are no hidden fees as long as you actually read the contract or pickup voucher. There will be charges, payable at drop off, which are not included in the quote. For example, there is a road tax at somewhere around 3€ per day, a 40€ fee for picking up at either a train station or airport, and an environmental fee if you select a car with automatic transmission. All of this is explained in the documentation.
There are basically two options for departing Paris for Normandy, taking either A13 or A14 which has an extra 9€ toll. Easier is probably A14, picking up a rental office somewhere near Porte Maillot, taking ave de Charles de Gaulle and preceding straight to la Défense and A14. Take a look at a map and I think that you will agree that it is very easy.
Alternatively, pickup somewhere in the 14th or 15th arrondissement, proceed south along any route to the périphérique, taking it west to Porte d´Auteuil and A13 to Normandy. You can pickup at Gare Montparnasse but you will incur the location surcharge.