Help with Japanese Postcards?

I'm in Japan, and I'm trying to send out a batch of postcards to my relations in the US. I've attached 80yen stamps to each of them. I hope that'll be enough for postage. I also heard that you had to write the word Airmail on the card somewhere? I've also included the addresses (and added USA at the end) on the stamp side of the cards. Assuming that that is all correct...I am still concerned about the boxes at the top of the card. I've heard they're for Japanese zip codes. Since I'm not sending them to a Japanese address (and my American ones don't fit) then I should leave it blank, right? And do I write the return address on this same side? I may have written the recipient addresses too big, so if I write my return address on this side, is it okay if it's a little off to the left? Or towards the bottom?

I use actual mail so rarely that I've become paranoid that all my postcards will get lost and never get to their destinations, so I'm just trying to be really thorough...