Has anyone created an import template for Inbound Shipments? I keep getting an error message that says 'Invalid purchaseorder reference key POxxx.' but I am pulling all the information from the PO so I don't know what's invalid...

Have you confirmed that you are mapping to the document number on the Pencil icon of the import mapping?

The PO field is a dropdown list right? That being the case you will need the internal id or in your CSV import may have to click through the mapping and specify you're supplying the text version.