[crmi]

I didn't see any request for a return envelope or stamp. Author decided to include them (unless I missed something).

[xp84]

It seems like common courtesy for me. Given the FSF are providing that service out of altruism, and are not the ones who neglected to give OP the license in the first place, it's only appropriate to not impose upon them to go buy stamps to send an international letter.

[crmi]

What about the paper, ink, labour involved? Why not slip $10 into envelope instead?