GST derivation on Freight and Packing & Forwarding Charges

As part of Oracle Financials for India - GST we have requirement as below in freight calculation. We tried various options but end up having issues in meeting one of the requirement. 

Request for solution & suggestions in meeting the below requirement

1) GST on freight (Composite Supply) should be applied based on the highest of tax rate of the item shipped/invoiced (For Eg: Assume that we Item A Tax 12%, Item B Tax 18%, Item C Tax 5%, Item D  Tax 28%. If we ship only Item A & Item C tax on freight should be 12%. Similarly if we ship Item A & Item B tax on freight should be 18%). Currently tax on freight is calculating based on the respective item tax slab and not highest of the tax rates of the items shipped