If I'm the SENDER :

I have to pay the shipping costs to send my letter to the Helper, of course, but...
I'm pleased because my friend the Recipient will receive something original.

If I'm the HELPER :

I have to pay the shipping costs to send the "inside cover" to the Recipient, all right, but...
I'm pleased because I have received a letter from the Sender that I can keep, and I'm pleased to help him (or her) by posting the letter he (or she) wants me to send to the final Recipient.

If I'm the RECIPIENT :

I'm pleased and totally surprised to receive a letter from a person I know but from a country that isn't his (or her) country ! I find this idea very cool, and I feel like I'd like to become a Helper too !

Finally, everyone is pleased ! :-)