This is aimed at the non-us visitors to Florida in particular.
When you've written all your postcards home and stamped them with the correct postage, you don't have to go to a postbox to post them, simply put them in the mailbox in front of your villa and put the red flag up and when the mailman does his next delivery round he will collect them to take them to the post office.
In my naiivety, I thought the red flag was an indicator that you had received mail so of it was raining you didn't have to go out and check and get wet unnecessarily, someone put me right, (well as far as mailboxes go anyway, ) a couple of years ago.

Babblin Boo