With all the talk about cards lately, I'd like to share an idea. I do this all the time, with varied reactions.
Buy a card for someone. Put a post-it note on the front of the envelope. Write the person's name on that post-it note. Put another post-it note inside the card. Write your message on that post-it (you might actually need more than one). This way, the recipient can just remove the post-it notes and reuse the card. So practical.