It’s hard to say what is the right thing to do... if the package was delivered to the wrong address, then the post office should pay what is insured. If they don’t accept responsibility for their mistake, Buyer/Seller should split the cost. As one party shouldn’t be fully liable in that case, the post office should. In your case, a locked mailbox, I feel like there should have been some kind of signature confirmation/ hold, as stated above, because the likelihood of a package getting stolen if dropped for all to see is much higher than at a private residence. (My small knife boxes get put inside my mailbox at my front door.) it seems though, you are surely not new to buying and selling with significant feedback, so this must be a one time thing? If it were me, I would surely go signature confirmation at a minimum moving forward. I don’t know that I could pay money, due to a locked mailbox delivery being stolen, if that is the case. Unfortunately, you will never know what really happened to your package. That information being a crucial factor in the recovery of the item/funds, it is hard to say who is mostly responsible, or partly responsible. Signature Confirmation should put every package in your hands though.
Edit: after rereading your post, it seems I misunderstood, and the post office says they put the package in your locked mailbox nobody could’ve stolen a package from! In that case, I feel like it absolutely couldn’t be your fault, and would split the cost with you. Do unto others as you would want done unto you. With your feedback, I would trust you, in this case. Obviously there are new members or some, that I would not.
USPS should admit they screwed up and pay what is insured!