Upvote:1
Generally you will pay the carrier your purchased from. When doing your online check-in, if you see the option of buying additional luggage, then you can do it there and just bring the receipt (although it will be on file too) when you go to the airport to get your luggage tags.
Now, if the option is not offered to you, you will pay it at the check-in counter which will be the one from Delta. Luggage fees are per way, so you will be 1 or 2 pieces (depending if your ticket had 0 or 1 in its included allowance). You will pay a similar fee on your return, either again when checking-in with AF or at the Air France counter, since they operate the first lag of your return flight.