Underwater post office
Look for a flag on the beach, which indicates that staff are on site, then dive in. Even if the staff aren't there, you can take the plunge and drop your postcards in the underwater mailbox. If you can't face the swim, Hideaway staff will take them down on your behalf, or staff at any local post office will send them for you. For obvious reasons, all stamps are specially embossed rather than inked, and postcards are waterproof.
Why an underwater post office? Well, Vanuatu is made up of 83 islands, and counts a great deal of water as part of its territory. Apparently it's beautiful down there, so the local tourist authority want you to dive in and see what all the fuss is about. And, just in case that doesn't do it for you, Vanuatu Post have also created a red-hot Volcano Post Office, 360 meters above sea level on Mount Yasur on Tanna Island. RM