Most cruise lines offer a concierge postal service or have a post box on board; something that is especially notable for those setting sail on a lengthy world cruise.
Passengers wishing to send post, including letters and postcards home, should pass them to Reception or Guest Services, depending on cruise line. All passenger mail will be passed ashore at the earliest opportunity and mailed via the usual postal services. Most cruise lines do a postal run into port, usually around 2-3 hours prior to departure, to give guests plenty of time to use the on board postage facilities.
The charge for using the postal service will vary from cruise line to cruise line, with a fixed rate added to your on board account each time you send an item. Postage fees will also vary depending on destinations involved on your itinerary and the final destination of your posted item.