By default, ShipStation does not display postage fees on labels unless the carrier requires it. A carrier may require showing postage due to the destination country (like Brazil), or due to service or carrier insurance requirements.
The USPS requires postage to show on the label in the following instances:
USPS First Class shipments using the Large Envelope/Flats package type.
When you add carrier insurance to the shipment.
To avoid this, use Shipsurance for your shipments instead.
In all other cases, the amount you paid for the postage will not print on the label. However, you can tell ShipStation that you do want the postage to print on the label. Choose Print postage on the shipping label for this shipment from Other Shipping Options before you create the label.