Help! Buyer is threatening chargebacks / paypal disputes against me for USPS Claims.

[Mijuchi]

Hi everyone,

I'm super worried right now and I need some legal help. In January 2018, I sold a figurine (anime figurine) to a young man. I told him that the figurine was in used, like new condition, and she came with her box and everything about her was fine. I insisted that the buyer purchase insurance for the item in the case that anything happens to her, and that I would package it responsibly, and that all damages would be issued to USPS for further notice. The figurine was shipped with these shipment details: Parcel Select Nonpresort® (2-9 days) // No signature required // Delivery confirmation required // Shipment is insured. 

The invoice itself had this written: 
This is a sale for Kotobukiya's Dark Angel Olivia. Upon sending in payment, I understand that I am paying for a USED Dark Angel Olivia and have negotiated with the seller (me) upon knowing her condition, which is used like new. Sending in payment I understand that the seller has purchased insurance on such a package and will package the item responsibly, but if damaged by USPS, the seller is not responsible for any breakage, lost packages, or damage to Dark Angel Olivia and such damage should be forwarded to USPS and redemanded for insurance money back. I understand that there is tracking on this package and insurance. The seller, me, will be taking photos of the item before it is packaged and then in the packaging, and then the package itself to ensure to USPS on the item's condition if USPS comes to damage the item (thus protecting the seller and buyer from "shady claims").

I sent out the figurine using silicone peanuts, a lot of paper, and more cardboard to prevent her from shifting. 

A few days later, the buyer emails me telling me that the box of the figurine arrived damaged. This was the photo the buyer sent to me:

 
(The crack running down the window of the box.) 

The box itself was written furiously with "fragile" written all over it. So I can't understand why USPS threw it around. Without asking me first, they went to USPS to file a claim. USPS told him that the seller (me) had to file the claim myself in order for it to be reviewed. When I went in to file a claim for USPS, they said that there was already a claim occurring for it, and there could not be two claims for the same case at once. Thus, we had to wait for it to be reviewed. He said that he would contact me later if there was an update. Months passed, and so I deleted the photos of the shipment off of my photo when I was cleaning data. The next time I heard from him was him cursing me out about how I "shoddily packaged it" and damaged his "waifu's house". 

A few months later, I notice that he did a dispute against me. He charged it as an "unauthorized charge". Paypal reviewed it. Closed it in 2 days. I contact him the following day after it was closed asking him why he chose to charge it against me for an unauthorized charge, he responded that it was because "I was responsible for the damage", even though USPS was the one who rattled the box, and the entire reason why I told him to file for insurance. Now, he's put in another claim against me for the entire $150 - this time as "item received as undescribed". 

Any ideas on what I should do to protect myself in this case?