Good news! Mail is slow but things will eventually get to their destination - most of the time...
If you are looking at sending anything and are staying in Havana then go to the Main Post Office (around the corner from Plaza de la Revolucion, across from Terminal de Omnibus) as you will have the most luck of your mail not going missing if sent from here. They also sell postcards and stamps as well. If you are staying at a hotel they will more than likely also have postcards and stamps for sale plus they will post them for you.
Most mail is also read before it leaves the country so make sure to keep things short, sweet, and upbeat. Think: "Arrived in Cuba, having a great time, wish you were here!" Even if it was horrible, lie. If you are sending a parcel, write in Spanish on the outside what it contains:
Spanish: FOTOS, REVISTAS, CARTAS, SIN DINERO
English translation: PHOTOS, MAGAZINES, LETTERS, NO MONEY
When it comes to mailing things, usually items going to Canada or Europe eventually show up but if you are sending things into the USA then it takes even longer and is more complicated. This is due to anything needing to go to the US being re-routed through Canada or Europe then making its way into the USA. That being said, when they do deliver it could be weeks to months after you've sent them and saying they might be damaged is a bit of an understatement.
If you are sending anything into Cuba, it cannot be worth any more than U$ 200 or else Cuban General Customs will confiscate it. If you are trying to mail cigars from Cuba to another country, you may discover them going missing....
