For those who have a japanese psn acct and have purchased a prepaid coupon to purchase gt5p.... here is a little help!! (pls be aware i do not read japanese! this is from trial and error)
1. when u click on the purchase icon when it becomes available u will most likely have 2 choices. (left) add to cart (right) purchase now
2. Click on the right side choice (purchase now)
3. The next screen i assume is asking if u want to add funds to yr acct, select the top choice, it will take u to credit card screen
4. scroll to the bottom where u have 3 choices.
5. choose the middle icon and click X
6. it will now give u screen with a further 3 choices.
7 choose the bottom icon of the 3 and click X
8 there u should be greeted with a screen to add yr coupon details.
9 Once you do this click the far right icon at the bottom of the screen.
10 This should be it!! you should have funds in yr wallet!!
obviously i have not purchased yet so there may be more to do after this but at least this will help you get this far........ good luck and i hope this is of some help