First, I'd consider that (1) I don't solicit such anonymous transactions, (2) I'm always short on bitcoins and need more, and (3) I keep my mailing address VERY private, and would immediately be suspicious if such an envelope arrived.
Now, assuming that the only things in the envelope were cash and a bitcoin address, I'd be stuck. There's no way to return the money. And if anyone is watching me surreptitiously, I'd have a hard time imagining doing the transaction would be seen as worse than just keeping the money (for now at least.)
So, yes, I'd do the transaction. It would take a little while, as I'd use the money to buy more bitcoins first, rather than send them mine. And I'd charge them quite a bit to compensate for the hassle and annoyance (refer to points 1, 2 and 3 above). But I'd probably take the time to also send them enough micro-transactions to form a message, something to the effect of "dont do this again" or "charging 50% next time."
The friendly hater
I laughed imagining a paranoid anonymous person trying to be friendly to a person that's somehow better at being
paranoidanonymous together with the urge of sending a message rather than just applying the high fee
