Shipping Insurance

Does anyone know if USPS has a limit on their insurance and does it cover things like precious metals?

Yes they do, but the limit depends on which service you use.
https://www.usps.com/ship/insurance-and-extra-services.htm

If you want it insured use a private carrier.  Even if they should cover it, you'll be fighting with USPS for months before they do anything about it. 

There are a surprisingly lot of items USPS, Fedex, and UPS will not insure. Look for a third party insurance company. I have a third party company who insures all my packages, the company I use also setup a sub Fedex account for me so I'm under their accoutn and get all their volume discounts which is a pretty big value considering theya re a shipping company and send a lot of stuff. look for companies who insure high end jewelry they tend to insure silver, precious metals and things often not insured by carriers. plus its easier to get a payout than dealing with uSps which is a pain.