I have been having this issue too....except I tried something and it seemed to work. Unfortunetly this isnt a fix but rather something is wrong with the mog station that needs to be fixed. I have 2 service accounts because a while back ago I picked up havensward twice. So I have had a spare account.
When I put the same code that read "this code is invalid" on my primary account on my second account it seems to work. I am thinking this may have to do with the storm blood expansion as my second account lacks the stormblood expansion (I bought the game over today on Amazon to see if it was the SE store code that was just having problems and sure enough the same results were duplicated) I did not use the activation codes on either of the games and going to see if I can at least return one of them if I can get either code to work. (I didnt get my preorder items either)
I hope they can solve this problem soon.