From the subscription terms and conditions:
Conquest wrote:• As a subscriber, you will automatically receive binders as they become available at £7.99 (€9.99) each. You will also receive special issues, though you will have the option to opt out of these beforehand.
They could perhaps have been clearer about the fact that 'special issues' are extra books that you have to pay for, and I couldn't see any mention of this anywhere else on the website from a quick skim.
However, this would probably be better off in the
ongoing Conquest thread, since it's not really news.