Ha! I just lost a hundred dollars just last week this way. Funny story - my aunt has a habit of mailing family holiday cards with $$cash$$. I am subscribed to “informed delivery digest” from US Postal service. I even saw the image of the letter I am (was) going to get. That was the last I heard of it!
Lots of people are employed by the Fed to handle these exact cases! I'd recommend getting it investigated, if nothing else, it'll help them keep justifying their jobs.
Mail theft is a federal crime. I would pursue this with them only if for the simple fact that if you don't, the next thing that gets stolen maybe your shiny new electronics being shipped.
