LOL amazon is funny.
I emailed them asking if I could have the promo code by paying the 3$ difference ( or substracting it from the promo code ). They answered me that it isn't possible, but they will refund my CK2 and I can then buy the package.
So instead of giving me a reduced promo code, they refund I game I already registered on steam so I can buy it again ( possibly having 2 copies then! )