When shipping with carriers such as Auspost, it's common for the actual rates to be different to the original quote. This could occur due to additional surcharges that were not known at the time of booking, or, it could be that Auspost mis-charged the item (happens).
Having a tool in which the invoice from Auspost, or another carrier, can be uploaded and then compared to what was originally quoted would be very handy. The differences could then be applied to the customers invoice as a credit or a charge.