Adding a percentage surcharge to an Invoice

HI,

Due to increased international freight costs we are forced to add a surcharge to all invoices we create in DEAR Inventory. This surcharge may change month to month, a little like fuel levy charged to us from local freight companies.

As this is hopefully a temporary measure we don't want to go through the time of changing every product price individually.

It would be good if there were an automatic way to calculate the surcharge in the Additional charges & services section, based on a percentage of the total of the line items.

I have attached a screen shot of my temporary workaround to try to explain what i am trying to do..