Locking Formula links from Other Workbooks

Can someone explain how to lock formula links from one workbook to another using Excel 2011? I will explain exactly what I mean. Please see the attached sample files and description below for reference:

I have already linked prices from "Price List Sample" to their matching items in "Invoice Sample". However, if I close the invoice sample and add an extra line in the price list sample, the rows get pushed down and the formulas no longer match correctly. For instance, if I add a line above the item, "Snacks" in line 2 of the price list, the formulas will no longer correspond correctly when the invoice is opened back up.

Is there a way to apply a formula link that updates accordingly when items are added to the price list?

Thanks in advance.