Also, be aware that you need to pay some taxes and fees when jumpseating back into the US. Going to SEA-CDG was just showing up to the counter and they made a listing for me (no charges leaving USA).
Returning, you have to pay some the fees at a special counter (not the check-in counter). Then they give you a receipt and you go to the normal check in counter to get documents to get you through security.
I think it was called the "Transfer Desk". But, not sure.