There is no direct domestic mail service from the U.S. to Jamaica. Jamaica is a sovereign nation (Commonwealth realm) with its own postal administration (Jamaica Post). U.S. domestic rates and services (First-Class, Forever stamps) do not apply.
[Recipient Name] [Street Address / P.O. Box] [City / Parish] JAMAICA
Postcards = "documents," usually no customs form. But include sender address. Prohibited: seeds, soil, certain publications.
Some think "Florida to Jamaica" is like "Florida to Hawaii." It's not. Jamaica is foreign. USPS treats it like any other international destination.