The information letter indicates that you have used the first free-flow motorway in France, the A79, for the first time but that you have not yet paid for your trip. 

Once you receive this letter, you have 10 additional days to pay for your trip, without any penalty! After this time, you will have to pay the fixed penalty. 

This system was put in place to help people who use the A79 for the first time. The next time you travel on the A79, you must pay for your trip within 72 hours.