Is there a way to insert line items via taPopRcptLineInsert onto an existing receipt?  When I try to feed it the desired existing POPRCTNM, I get the error 'Input variable contains a duplicate document (POPRCTNM)".  It works of course if I leave the POPRCTNM field empty, but it places each separate transaction on its own receipt and I would like to manage the receipts better than that.  Any ideas?

Not that I know of.

SOP Transactions have the 'UpdateExisting' property, the fact that there is nothing like this in POP Receipts makes me think that it's a one time thing. I don't see any functionality to delete a POP Receipt so you can resubmit it, either.