I've started sending paperbacks instead of greeting cards when someone I know needs a get-well-soon card. In stores around here, greeting cards are often $7ish + postage. I can frequently ship a paperback with a gift receipt for $5 total. I include a gift message on the gift receipt, and choose a book I think someone might like to read while they're out of commission.

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.