By default, ShipStation does not display postage fees on labels unless there is a carrier requires it. This may be required due to the destination country (like Brazil), or due to service or carrier insurance requirements.
In all other cases, the cost you paid for the postage will not print on the label. Instead, you can tell ShipStation that you do want the postage to print on the label. This setting is located in the Other Shipping Options section of the Order Details.