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Couldn't buy my game on galaxy but it worked on with same credit card info.
game : Season-Pass Witcher 3
OS : W8.2 x 64
Localization : France-Strasbourg

From what I can tell, it looks like this wasn't related to Galaxy, but rather it was some random technical issue between us and our payment processor. The issue seems to have resolved itself when you tried again.
Sorry about the original issue, though!