Commission report - how is a rep expected to link a inv. and s.o.?

We have a schedule that pays on collection based on the Invoice and another schedule that pays on the ASA which is a Sales Order based commission.  My reps don't care about any of this, they just want to see what they were paid for each of their orders.  In other words, they want to see the combined invoice commission and Sales Order commission.

I have been trying to customize a Paid Commission Detail report to show the Created From field from the Invoice so they can at least associate an Invoice and a Sales Order but this doesn't seem possible.  As ridiculous as this is, it appears at this point I cannot ouput a report that shows the total commission for an order, even if I DO post-process it in excel because I can't link or join the invoice with the sales order.