It's a glitch in the purchasing process. The first thing recognized is
the normal price of the club, then your credits balance, the discount is
brought into the equation after that. Even though in the end, you get
the discount, you have to have enough of a credit balance for the full
price of the club in order for the transaction to begin processing and
eventually get to the discount being applied.
CS will put the purchase through for you. Click on "Contact" at the bottom of this page.