Actually it worked out great. My friend absolutely loved getting the postcards with stamps from each island and postmarked there as well. You can get the stamps for the different ports at the pursers desk or when you get off the ship at each port. There was usually a booth where I could pick up stamps and mail things off from there. St. Thomas had a post office in the shopping area near the ship.
The ship has free postcards at the pursers desk as well that show the ship and list Carnival on it. I thought it was appropriate to send off saying.. I'm here your not.. naaa naaa naaa. HAHAHA Joking.