The tax is inherited from the customer, already at the point when the charges are transferred to the order. This is because typically in the US sales tax is determined from the point of where goods are delivered. As this usually is the customer location this means that the customer tax group is a good indicator for calculating tax.

Transportation module therefore won;t be able to cater for Australian taxation system. Vendor invoice is the accountable document and should have the final say in what GST is applicable. TMS doesn't take into account any GST data that is entered, it seems to calculate GST, where TMS should be allowed to take into account "Actual GST" as per the invoice.