Are you logged into your SD account on the SD store page? If you're not it'll give you that error. There is no option that I saw to log in as part of the process.
Log in first, then try to buy it.
After adding your item to the cart, before clicking "Checkout", please go to the upper right hand side of the page and login with the same credentials you did on fallenenchantress.com. You should be able to proceed with your purchase from there.
Sorry for any inconvenience guys, this will be looked at ASAP.
