Try the three numbers from your postal code followed by 00
Example p9n 3x1 would enter as 93100.
Japanese postal codes are ###-####, so I wonder if there's an equivalent trick like entering your 3 numbers followed by -0000.
I set up an Amazon Japan account with Japanese address and Canadian billing and managed to order an eShop code, but I'm still waiting for pending verification, which it says can take up to 4 hours. Fingers crossed that it goes through.
