USPS "Unclaimed" Item?

So I won an auction on Wednesday Afternoon for an 8ight-T rear diff and it was shipped via First class mail the same day. I was just looking at the tracking today because I thought that it would have been here by now or tomorrow at the latest and I came across this:

Class: First-Class Mail®
Service(s): Delivery Confirmation™
Status: Unclaimed

Your item was returned to the sender on December 18, 2010 because it was not claimed by the addressee.


Detailed Results:

Unclaimed, December 18, 2010, 10:43 am, SAN DIEGO, CA
Arrival at Post Office, December 18, 2010, 8:49 am, SAN DIEGO, CA 92130
Processed through Sort Facility, December 18, 2010, 5:00 am, SAN DIEGO, CA 92186
Processed through Sort Facility, December 15, 2010, 6:59 pm, PHILADELPHIA, PA 19116
Electronic Shipping Info Received, December 15, 2010

I've been having issues with USPS around here but on Saturday I was home all morning until 1 or 2 PM, never heard a knock or a doorbell ring and never got a delivery attempt notice on the door either. I was wondering why this might have happened? Something similar happened a couple weeks ago when Mike sent me some Mod 1.2 pinions for my Baja. I never got a delivery attempt notice and they just kept it at the Post Office after one "supposed" delivery attempt.

So do I call USPS or the vendor and have them reship me the item? Will I have to pay for shipping again? This is kind of frustrating...