Labels are charged to your account once they've been downloaded and used, and are payable once an invoice has been issued. 

At the end of each billing period, you will receive a comprehensive invoice for the services - including the labels that you’ve used throughout the previous month. Once an invoice is issued, you will receive an email with a breakdown of the services you have used. 

From this, you can review your shipments and downloaded labels against your invoices and if necessary, raise any queries that you may have.