Inbound Shipments are... insane. I have other words, but let's go with that. They're not transactions, not quite lists, but a weird hybrid placeholder record with valuable information.
There's no way to link an Inbound Shipment to the PO. Well, you can read the 'solution' at #79543, but it's awful at best.
I've tried a few things w/ inbound shipment saved searches and workbooks to get reports done, but without scripting, I don't think there's a good solution right now to do anything with the actual records.