Help with self addressed envelope (US to France / France to US)

[Noxend]

Hey guys so I’ll be sending my first bit of fan mail and I need some help. I’ll be sending an envelope from the USA to France. I want to know how to go about including return postage from France back to the USA. How do I obtain French postage and how do I know what is a sufficient amount? Obviously want to get this right to make it a painless process for who I’m mailing to. Based on what I’ve researched, do I need to purchase a French intentional stamp? If so is the only assured method of that to buy from Laposte and having it mailed to me in the US? Any advice is gravely appreciated.