By default, ShipStation does not display postage fees on labels unless there is a requirement in place by the carrier, such as including USPS insurance.
When printing a label in ShipStation, you can make sure you are printing the label without showing how much you paid for postage. Simply click on Orders, and click the order number that corresponds to the one where postage needs to be hidden.
In the Order Details pop-up, look for the Other Shipping Options section on the right side.
Now, make sure that the checkbox next to Print postage on the shipping label for this order is unchecked.
Close the order, and now postage paid will not print on that order!