Sorry for the delay on the response.
There is a different option to do what you are describing.
First, go on Purchase Orders module, and on Settings, open "Purchase Order Settings"
Check the checkbox "Extra Costs Invoices from different Suppliers".
Usually when you select the 'Create Purchase Invoice' option from a Purchase Order, the Purchase Invoice will be created immediately, and it will only contain the ordered Items. If you are using this option, the Purchase Invoice will not be created immediately, and instead a window will open where you can specify that any or all of the Freight, Customs and Extra Costs are to be included in the Invoice as well. This window will also allow you to create separate Invoices for these Extra Costs, perhaps using a different Supplier to the one on the Purchase Order.
This option is not valid if you create the purchase invoice from the goods receipt. If you do it from there, the purchase invoice will be created automatically and it will include all extra costs, like it does now.
I will add this functionality as a wish for future releases.