Returned an item on Amazon, item was lost on the way back. All parties initially denied responsibility, but now two are giving refunds

Hawaii

I bought an item on Amazon from a third party seller. Got the item, in working condition, but decided it wasn't for me and sent it back, well within the 30 day return policy from the seller. Since my money was with the third party, I didn't file a claim through Amazon and went directly to the seller. Sent the item back, insured, through USPS. USPS claims it was delivered to an agent, Third Party initially said they had it, then changed their story to say they never received it. Ended up having to ask Amazon for a refund almost a month after continually being stonewalled. Third Party said to file a claim with USPS, USPS said since it was signed for and was not their responsibility even if it was insured.

After three months of everyone denying responsibility, hours on the line with various customer service teams, Amazon issues me a refund. However, USPS agreed to a refund too. How do I go about this? At this point, I believe everyone is at fault, but I'm worried this is fraud. I'm more than happy giving one of the two their money back, but I don't know who is at fault actually. USPS for handing it to a stranger, potentially, third party for losing it and maybe lying about it, or Amazon for doing business with them in the first place. Should I keep the money? Return it to someone without knowing which party is legally at fault? Please help! Thanks!