You buy the cards, write on them, address them and go to the post office.
And in Cuba, that's where it gets interesting.
The first clerk takes one look at the cards and indicates you should go to the second clerk. The second clerk asks where the cards are going. Three to Canada, one to the United Kingdom, you say. She shrugs like she doesn't know where that is. "Angleterre?" you say hopefully. She says nothing to you but there is a steady and loud stream of chatter to the other two ladies at their wickets. All three look put out by the imposition of this foreigner. "You must take them there," she points back to the first clerk. The lady makes a great production of locating two 50 peso stamps from the copious folders in her plastic bin. She clucks her tongue in exasperation several times. Then she jots down some notes on her scratch pad, attaching the stamps to the card with a paper clip, and passing them, along with a nail polish shaped bottle of glue, back to the first clerk who busies herself with attaching the stamps. She repeats the process with the Canadian cards, each of which require one 65 peso stamps. There is more jotting down of numbers, head shaking and tongue clucking.
"Cuanta cuesta?" you ask. "One Eighty," she says carefully in English. You pull out your money. There is great consternation. No,no,no. Only national money. You have no national money. More over-the-counter talk with the clerks. More head shaking, eye rolling and apparent disgust. The people behind you get involved, a handsome elderly gentlemen and two young women. The elderly fellow shows you the national currency. You try to give him your money in exchange. He won't take it. The clerk is about to hand you the cards, then pulls them back, then hands them to you, shooing you away like she's done with all this nonsense. "Finished," she says.
You're so confused. One of the young female customers speaks. She points to the old man who smiles a beautiful smile. "He has paid for you stamps."
"Muchos gracias!" you exclaim.
She takes you gently by the hand, and you walk out to the sidewalk where she takes the cards and deposits them in the mailbox with a flourish.
I don't know if the cards will reach their destination, but they've had an expressive send-off.