I believe the vendor to be at fault. I do a lot of mailingfor eBay sales, and use the USPS for my shipping. I have had only one problem, and that was a part that was lost in Puerto Rico. All of the shippers lose packages. The USPS is the only organization that publishes their on-time delivery rate is the USPS, and the numbers are in the upper 90s. (That said, I do have some issues with the scanning procedures the USPS employs, and have voiced my opinion several times. Like someone said above, "The squeaky wheel gets the oil.")
Next, I believe you said you spent $30 for shipping. That is way more than it cost to mail the parts via Priority Mail, therefore the vendor had sufficient funds to ensure he protected you and his company - greed got the better of him.
Finally, it is the vendor's responsibility to deliver the product to you. He has all of the shipping information and receipts and is the one who should be trying to determine what happened to the delivery.
That's my story, and I'm sticking to it.