I’ve already paid this “one time” fee.
Contact us via email using the account and billing link on our support page and we can work with you on account-related issues:
Each version of classic had its own fee: classic (era), TBC, and WotLK. You only need to pay each one once for each classic version of the site that you wish to use, so we’ll get you sorted out if it is asking you again for the same version for some reason.