It sounds like you want to activate the Duty feature in the program so you can spread the extra costs over each line item based on percentage or dollar amount.

Duty can be activated under Setup, Settings, Payables, Duty, then on each vendor, activate the Duty option on the Options tab.  This will add two new columns on the purchase invoice when you select that/those vendors.

That is great news, will explore that option pronto and let you know how it goes.