Club Penguin have now added 3 new postcards! They are categorised as “Featured Postcards”.
(click to enlarge)
The new ones are “Long Time no see”, “You’re Funny” and “Turn Green”.
To send a postcard, simply click “New Message” in the top left corner of your mail, then choose a buddy, and finally a postcard to send to them. It costs 10 coins to do so.