Please be sure to go to Castaway Cay Post office as soon as you get off the ship, rather than on the way back.
We were deterred by the long line,so we decided to go on the way back. Unfortunately, they had run out of the postcards that they sell in the post office...so we couldnt mail any.
However, if the line is way long(it also moves ssssssslow)My advice is to either bring postcards with you from the ship, or buy some at the gift shop on CC. That way you'll cover yourself if the line is too long early, and you decide to go on return trip...at least you'll ensure that you won't be shut out!!!!