Several people have reported being unable to purchase premium packs after having activated the free-to-play version of EC.
The solution to this is simple; all you have to do is delete the free version from your Steam account.
You can do this by taking the following steps:
1. Select "Steam Support" from the "Help" tab at the top of the Steam client window.
2. Click on "Games, Software, etc." and find the appropriate Eternal Crusade product.
3. Click on "I want to permanently remove this game from my account."
Rest assured! Characters and progression will not be lost due to this, as it is your Steam account itself to which your game data is tied.
We apologise for the inconvenience and thank you for your patience!