Chiming in with doctornerdingtonpaper? Why not send them a draft of your desired response too (this is hyperbole/sarcasm, I don't think you'd actually think/do such a thing). And with chestnut_pod that sending just a stamp is a lower-key way to achieve the actual desired effect, of removing hurdles to make it easier for the correspondent to reply if they choose, without presuming anything about their potential reply.