Use Idoc Type: INVOIC01 and Message Type: INVOIC
In WE20 define Inbound params for your Vendor (type:LF)with process code: INVL – link this with FM: IDOC_INPUT_INVOIC_MRM.
Make sure you got a Vendor with a reconciliation account in XK03. Then do all the EDI “financial settings”, i.e. OBCA, OBCB, OBCC, OBCE etc.
Make sure you map the PO number and it’s line item into E1EDP02 segment. SAP performs a triangulation between PO item/Goods Receipt/IR and each line is treated as a unique item.
For Idoc layout, check WE60 or even better: ifr.sap.com