How to get my letter returned back?

Hello, I am a student living in french.

I am going to send a letter containing important documents in japan and I would like my receiver to return me these documents back. Since I don't want him to pay for that, I would like to put a stamp on a enveloppe that I will put into my first one so that he would just have to put it into a mailbox.
However, french post service, La Poste doesn't provide stamps which allowed to do that. I would like to know if there is a possibility to buy japanese stamps which works from Japan to France on the internet, or if some business provided this kind of services ? I was thinking about EMS or Yamato but I don't know what are the specifics words in english and in japanese so it makes it difficut for me difficult to find informations about it.

Thank you in advance for your replies ! :)

If you are only need a few documents returned you could make use of an International Reply Coupon:

http://en.wikipedia.org/wiki/International_reply_coupon

Buy appropriate pieces of International reply coupons at La Poste.
http://timbres.laposte.fr/af/bv/core/content/content.do...

When you buy a piece of coupon, he/she brings it to JP post office, he/she can be exchanged to a 130 YEN Japanese stamp. If you send two coupons, he/she gets 260 YEN stamps. So that you need to buy and send appropriate pieces of coupons and tell him/her to bring the French coupon to JP post office for exchange.