Customer says package wasn’t delivered, need some FBM experience advice

I don’t have much experience with FBM sales so I could use a little bit of help here from some of you more experienced sellers.

I do FBA, i’ve been doing it for about two years. I’ve done FBM a few times for a handful of products throughout these two years, but it’s not something I do regularly. but the last couple weeks I have been dipping into some FBM because of a great product that provided the opportunity to make some quick sales. It’s all gone very smoothly except today I received a message from a customer saying that their item was not delivered. They are requesting a refund.

I checked the tracking and it says it was delivered to a USPS parcel locker on August 30. Is there any way for me to confirm if that package has been picked up? If it hasn’t and the customer is not willing to go get it themselves can it be returned to me?

I find it a little strange that they’ve waited this long to do something about it if it was truly never received. They’ve had the tracking information the entire time obviously and could see this for themselves. so if they’ve known it was sitting in a parcel locker for almost 2 weeks, It’s weird that they haven’t done anything about it yet. But if they’ve had trouble actually obtaining the package for some reason, they haven’t said so. Their message simply said it was a refund/return request, and the text was “ did not receive package.”

I’m sure I’m overthinking this but as I said in the 150 or so FBM packages I’ve done in the last two years, and maybe 100 eBay packages, This has never come up and I want to make sure I handle it correctly.