A user may want to have a commercial invoice printed for international shipments but not charge the customer anything for a particular item. A commercial invoice must have a rate, or it cannot be printed and will generate the "Invalid Customs Value" error.
When an item is added on a sales order, the rate automatically sourced in would be the base price on the item record. This can be modified at the sales order level to any other rate as desired. This rate is the one that will be printed on the commercial invoice. The total amount charged can be $0.00 and, as long as the item rate is greater than $0.00, the commercial invoice will still print.
This will result in a commercial invoice being generated without the customer being charged anything for the item.