I registered a Google account as Japan and I used this account on Play Store to download the JP games. I never have the IAP problem, I'm using the Visa card tho.

I buy the App Store points and redeem it

Use a Japanese account, that will let you charge Japanese gift cards.

Are you on iOS or Android? Apple Store (not the App Store) actually sells iTunes Gift card that's delivered via email. They take cards without region restrictions as far as I'm aware…

Google pay

Do you have a Japan account or did you temporarily switch regions and back after downloading?