I know that if I Prepare Invoice FROM the Sales Order after the shipment and return are complete, I will get a single invoice with both the return and the new item(s).
However, that isn’t normal processing. We don’t want to handle RMA invoicing separately. When we use Process Shipments > Prepare Invoice - we want to hold off invoicing till everything is returned and replacement shipped and create a single invoice (or credit memo if more returned than shipped).
Is there any way to force the shipped and returned items into a single invoice?