Hearing this wonderfully romantic song on the radio reminded me of how technology has changed the world that we live in so much. Sending a letter by post (snail-mail in modern parlance) is now considered prehistoric - a 'love letter' would most likely now be sent by email, yahoo messenger, text message, or via twitter or facebook.
Actually how many other "letter" songs can you think of? Here's just a few: 'Return to Sender' - Elvis. 'The Letter' - The Box Tops. 'All My Loving' - The Beatles. 'Please Mr Postman' - The Carpenters.