When you attempt to purchase a product where the total cost is greater than the amount in your current credit balance you will see a message saying 'Insufficient funds'.
My balance says I have more credit than the product I'm trying to buy
Your balance is composed of two separate amounts, a 'real money' balance and a promotional balance, usually awarded when you redeem a voucher. Real money balances can be used to purchase any item--including mail order books and telephone subscription services--whereas promotional balances can only be used for Race Passes, downloads or other purchase consumed directly on the site.
Your account management page shows how your balance is currently composed.
If you're still having problems, please contact us for further assistance.