As a developer, ask the Dynamics users how THEY would do it. So: after creating a PO, what happends next in the process?
There are several paths, depending on if they want 'shipments' or 'shipment/invoices', and I'm not going to go into that here. Ask the users what type of doc comes next, they look up the eConnect doc that creates it.
This will probably be some type of receivings document.