There are a few ways you can view what you were charged for a specific label.
Check the Carrier Fee column in the Shipments Grid.
If you do not see the Carrier Fee column, go to the Columns drop-down menu and check the box next to Carrier Fee to add that column to your grid. ShipStation adds new columns to the end of the Shipments grid, so you may have to horizontally scroll to view it (click and drag the column to move the column to another location).
Open the Shipment Details screen - this can be done from either the Shipments grid or from the Order Details screen - and view the label details panel. The label cost is next to the shipment tracking number.