If I'm the SENDER :

I mustn't put several envelopes in the one I'm sending to my Helper : one envelope in one envelope only, please.

I mustn't choose a Helper who is living in my country.

If I'm the HELPER, I'm just a "relay" :

I mustn't open the envelope I have to forward to the Recipient !
I just have to affix stamps of my country on it.

The Recipient must receive the letter as it was sent by the Sender : only stamps must be added, and, why not, an Air Mail label.

If I'm the RECIPIENT :

Nothing ! I just have to be happy and surprised to receive a letter coming from an original location !