Are you creating the Sales Invoice from a Delivery?  We put the Customer PO number on the Sales orders in the field "Customer PO No.".   Then when we invoice from the delivery, that number appears on the Sales Invoice in the field "Reference" in the header.  Is that what you are looking for?

Are you creating the Sales Invoice from a Delivery?  We put the Customer PO number on the Sales orders in the field "Customer PO No.".   Then when we invoice from the delivery, that number appears on the Sales Invoice in the field "Reference" in the header.  Is that what you are looking for?

we do enter the Customer PO No on the "Stock" section of the invoicing screen, and that shows up on the invoices properly.  it's finding somewhere to house the value of that PO that is bugging me.

Is there a reason why you cannot simply join to the sales order header table using the Customer PO No on the invoice as the link (provided your customer PO numbers are always unique) in your report? The sales order header table contains the totals in fields ORDATI (total with tax) and ORDNOT (total without tax)