I agree in general, but as a one-off thing I'd very much enjoy getting a lime with a message saying "this was cheaper than sending a letter myself"
I guess it's a bit like postal arbitrage, if I accept the cost of greeting cards themselves as part of the cost of the activity.
To the extent that anyone has commented much, those who have commented had very positive reactions to what amounts to a book recommendation and a copy of the book I'm recommending along with a little note.