This will depend on whether your label is from a ShipStation Carrier or from one of your own connected carrier accounts.
For labels using ShipStation Carriers, ShipStation is considered the postage provider. The first time you create a label with one of these carriers, you'll be prompted to enter the payment method to use for your label fees.
For labels using your own connected carrier accounts, that carrier acts as postage provider, not ShipStation. The label fees for these labels will be charged to you by your carrier and appear on your regular account invoice from that carrier.