(If this is the wrong place for this, feel free to move it)

I've been playing the trial for the last two weeks, and have decided to buy the game. However, when I enter the additional information (which I'm assuming is used for billing), all of the information is accepted, except for the post code; it says its invalid. My post code is correct, as services like Steam accept it.

Is there something I'm doing wrong? Help would be much appreciated.