Try using a different search engine like google or firefox.
MysticHaze means try a different Browser.
Or temporarily allow CoE to use popup/scripts. This seems to be the main culprit (But when someone tries a new Browser they normally haven't turned those things off yet so it normally works).
There is an option on check out to choose to apply merchandise credit you may have got through KickStarter. Are you trying to apply more credit that you have access to?
I am not sure the credit from recruiting players is applicable to this option, so perhaps it should not be added here.
I never took part in the kickstarter, i got store credit from my baron pledge once but that was spend long ago.
Besides it wont let me get to a point to where i choose how i want to pay, so im not sure whats going on.
Tried several browsers, it isnt working for some odd reason and as a result i can neither upgrade my pledge nor buy any additional EP.
I guess ill have to settle with what i have.