If you've exhausted the allocated lookups under your plan, you can still continue to perform Additional lookups.
Additional Lookups are enabled automatically to prevent any interruption of service. The cost of Additional Lookups varies based on the plan you are subscribed to and can be viewed in the billing section of your account page.
If you wish to disable Additional Lookups you can do so by toggling the 'enabled'/'disabled' switch on the billing section of the account page
Any accrued Additional Lookups will be billed automatically at your subscription renewal date.