Thx, but this isn't the problem @Bryan_M .   All of us posting here know how to create a re-billable charge, and rely heavily upon its functionality. 

The problem is that the supporting document (pdf receipt or vendor bill) that's attached to the rebillable charge doesn't stay attached when we add the Rebillable Charge to the Client invoice.  The most important aspect of having the attachment in the first place is sending the supporting proof to the client.

Many clients will not pay the invoice without proof (receipts or pdf of the vendor's bill).