Custom Item Description not showing on report

At my company, every order we ship has a unique part number set by the customer. When we create an invoice, it's set so that we enter an item on the invoice, the description automatically begins: "Customer Part Number - " and then we write in the rest. So the invoice says "Customer Part Number - XYZ123" 

I have a customer who's requested a report listing all of his invoices and the part numbers.

I've included the Item Description on the report. The report only shows the words "Customer Item Number" as the description - the actual part numbers we added after that aren't showing. How can I include the WHOLE description, including the part number, on the report? 

I've tried adjusting the Detail Level on the report filter.

I can't go back and write the part number in the memo field - one, waay too many invoices are involved, so the time would be prohibitive, and two, we use the memo field already for something else.

Any help would be most appreciated! :)