I'm in Japan and I want to send some postcards, but somewhere I read that I can get charged extra for writing in the address field (which I suppose is the side with the little squares and the space for the stamp), I really need to know about it, and I am also wondering, do I have to put the post code in the squares? or just jot it down with the rest of the address?
The side which has 5 squares is for the recipient's address and your address.
5 squares are for the zip code.
A=recipient's address (you can write it horizontally)
C=date (can be omitted)
D=your address and your name
The example on the right side on the site I linked is an example for a picture card that you can't write your massage on the side that a picture is printed.
You draw a horizontal line to divide the address from your message.
Please make sure that do NOT decorate the side for the address. It could be cause of extra charge.
That's more for domestic post cards that probably have the message on the other side. For Tourist post cards I really don't think it would make too much difference.
The biggest challenge is finding post cards, at least good ones, in the first place! Japanese use post cards more for things like New Year and advertising not for sending "wish you were here" comments to friends back home. When we go on holiday my wife never sends post cards home whichI find a little strange...