Since there's a suggestion that we can also leave messages on the official message board, I think it's a good idea too. Japanese fans are asking overseas fans to send postcard because Johnny's takes real postcard more seriously than online message. If you have trouble to send a postcard, then you might try the message board to state your wish. Maybe you can add a picture of the Jin/KAT-TUN/Johnny's goods you have (a tinypic link or something) to make the staff-san think more seriously about your message^^.
official message board
(I think you can either write Japanese or English. If you are not sure about japanese, check the sample by clicking the link of the postcard one.)
For people you would like to send a postcard, check here
The timing is really important as Jin's solo live will finish in two weeks. Let's do it as soon as we can.