France has dropped test for vaccinated arrivals. You need to test the day before flying back to the USA to meet American entry requirements. You can do an emed antigen test or get tested pretty easily at french pharmacies and test sites for about 30 euros or less.
Not sure if you can get passe sanitaire on line. They started and stopped. You can look up a pharmacy in your neighborhood that is authorized to do it. Costs about 30 and takes 10 minutes.