If you have previously purchased some Prepaid Credits, you will be able to redeem them when purchasing any item or paying for any fee.
Please find your way to the start of the checkout process (links to which are usually found in your invoice). On step 2: "Your Cart", go to the Redeeming Credits section of the cart. There you will see how many are available, and you can specify how many you would like to redeem. Afterwards, continue with the checkout process as usual, and once completed, the credits will be taken from your account and applied to the payment.