I don’t know if this is a bug or not, but it now costs money, not coins to increase postcard storage. I just bought a lot of coins but can’t use them. I wonder if it is a bug because my wife who has a Japanese account can still use coins to increase postcards storage.
That’s right, I’m not sure why. If you want to continue purchasing storage at that point a better bet would be to go to the Web store and buy the postcard collector pack. If you contact support they might refund you for a coin purchase, but it probably depends on the person you get for support. If you are spending money to collect postcards it’s worth while to purchase the pack regardless though.