That is such a tough one and there’s no real answer to your question. Technically he probably fulfill his obligation unless you specifically requested something other than what he did. Now you’re out money and don’t have your parts, that sucks.
I know it’s little consolation but I’ve learned this lesson the hard way from both sides. Last year I sold a Comer part on ebay and shipped it by USPS. The buyer claims he never received it (no way to prove either way) and PayPal gave him a refund which I had to reimburse even though I have a receipt from the post office. Fortunately it wasn’t that much money but what pissed me off royally is the wanker left negative feedback on me on ebay. He got his money back and I have a very, very long history of positive feedback so I’m at a loss as to why he would do that. I’m just hoping karma catches up some day. I’ve also had similar situations as you’ve just experienced but usually I don’t receive anything at all.
Now days I only ship and have items shipped to me via Fedex or UPS so they can be tracked. It may cost a bit more than USPS but you always get what you pay for.