My suggestions: the car service if you have the bank. Uber may be problematic with a strike as the French are less inclined to stand by and watch during strikes! RER train downtown is very easy. Just look at maps and pick your most convenient metro to the Bercy. You can buy tickets with any credit card or cash at the airport train station "in" terminal 2 which is Delta and Skyteam. Or you can use a machine to purchase your ticket, but you will have to use a chip and pin card--hopefully you know your pin for the corporate card, or have a personal chip and pin card.