You are correct in stating that no duty free is allowed to or from SJU. However, there is a provision at the duty free stores in SJU that will allow you to buy local rum at very low prices. Even lower than what you would pay at the local stores outside the airport. I am not sure how the ability to purchase alcohol at duty free has been affected after the recent security fiasco dealing with liquids. I do not think the store delivers to the door of the aircraft - you used to take the bottles with you after purchasing them.