How to customise Orchestration process for reservation and invoicing stage

There are 2 subinventory "STAGE" and "BONUS". while the sales order is getting issued for an item. 1st scenario sales order is issued to a customer 10 quantity which it has to go from the STAGE subinventory - for the invoicing part it tax and the unit price amount must be calculated. For the 2nd scenario the 3 quantity from BONUS subinventory is issued free - for the invoicing part only tax should be calculated there must be no unit price. How to customise the orchestration process for the sales order issue!