This will depend on which carrier or postage provider you are using. You'll either be charged when you create the label in ShipStation or you'll be sent an invoice you must pay each month.
Most carriers are considered post-billed carriers, meaning you are charged for the labels you use on a monthly invoice cycle. UPS and FedEx are examples of post-billed carriers. Post-billed carriers also only charge you for labels you actually use, so if you create a label in ShipStation but don't use it to ship a package for whatever reason, you should not see that label appear on your monthly invoice. ShipStation recommends you void any labels you don't use so that your shipment records reflect your actual shipment records accurately.
Some providers may automatically deduct label fees from an account balance managed by the provider or charge the account credit card either when the label gets created or at some other regular interval. ShipStation cannot provide details on how your provider bills your specific postage provider account.
If you are unsure if your provider charges at the time of label creation or bills you monthly on an invoice, contact your provider account manager for confirmation.