There's no extra fee when you setup pre-authorized payment on your account. You only pay for the services you have used! Have you taken a look at your invoice to see what the charge is for?
It's good to know that if the card is changed or added in an on ongoing cycle, the withdrawal will only begin as of next cycle. Our system takes a full cycle to reflect the updated informations. In this regard, you would have to make a manual payment the first month of registering to pre-authorized method.
Hope this clarifies!