Seemingly lost USPS shipment; no case opened yet

I haven't seen this exact situation yet, and eBay support's response was ambiguous, so here it is.

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I shipped an item to a Michigan buyer, first class, within handling time, with tracking information, on May 17th. The ETA was May 20th. It made it to the Oak Creek, WI dist facility on May 19th. That was the last entry until May 23, when "In Transit, Arriving Late" was noted. There are no entries since then. I didn't realize this until the customer contacted me a few days ago, saying he didn't receive it-- but he didn't open an eBay case yet. I immediately opened a USPS lost mail case. He contacted me again yesterday, and since the USPS hadn't given me new information I called eBay. "Ryan" confirmed that my seller protection would apply here, but he didn't give me a path forward so I'm leery of doing anything based on a verbal communication. I'd hate to tell the buyer to open a case against me just so I can get this closed out-- because eBay may betray me as they have often done to their sellers. At the same time, this person didn't ask for this nonsense, and deserves to get his item or his money back promptly (my seller rating notwithstanding). Has anyone had this happen, and how did you handle it?