When you order online, an authorisation request is sent to your card issuing bank to ensure that the funds are available to make payment.
If everything is ok, your bank will place a reserve on your account for the amount of the transaction.
This will allow the funds to be taken from your account for the transaction.
If the transaction fails after authorisation the funds will still be reserved on your account. Although they have not been debited from your account they will appear to have been taken. The bank will release the hold and you will not be billed.