Any tips for dealing with the US Postal Service?

Seems the Post Office delivered 1/2 of my package to its destination. I had Part A and Part B wrapped up separately, and then again both into a single box, which I felt was quite up to the job, but somewhere along the line the box failed. Part A was re-boxed, shipping labels transferred (by a postal worker somewhere), and arrived at the destination. Part B, without which Part A is useless, is MIA somewhere between PA and CA.

I understand I am probably out of luck, but intend to file a claim for at least the small insurance value I put on the package.

Discuss.