Add cost to Finished good before Invoicing.

I am curretnly using NAV 4.02

I am trying to setup costs to be added to the finished good at the time of invoicing. We create custom Labels (specific to the Customer placing the order) that are custom printed and added to the finished good after the Sales Order has been entered.

Here is an example:
Finished Good A is on a Sales Order.
Label B (Needs to be custom printed based on demand from a Sales Order specific to the customer)

Currently we have Supplemental Items for Finished Goods that consists of only Label B. We would like to incorporate the ink and labor costs of printing the label to the finished good so the cost is rolled up into the Finished Good when the Sales Order is Invoiced.

My question is if I should be using 'Item Charge' after the Invoicing, or is there a way to add Resources for the ink and labor to accomplish this task. If not, what other solutions do you suggest.

Amy