Just for your knowledge:

On the Air France site in France, you can depart only from french cities.
If you want to depart from US citie, you need to go to the US site, but then have to pay with an american credit card.

On the Delta Airlines site, if you are french, you are only allowed to depart from France, which is still worse, because you are even not allowed to buy the flights for South of France, or Germany in codeshare, you can only by intercontinental (AF monopoly).
If you want to buy a national US flight, you need to pay with a US card.

Few sites seem to allow to buy any ticket from any country with any credit card. at the end, I do think the credit card is the problem.
One of these sites that was pointed to me on this forum (but I did not really use it, so be careful) is www.flights.com