I think USPS Registered Mail is the only service where insurance will be honored on coins/currency. With FedEx, UPS and other services from the USPS you're SOL.
You didn't give any details so taking you at face value it's time to file a complaint with the Postal Inspection Service.
While these losses are deeply unfortunate, I will say sending expensive gold coins from Carson City to Reno, a mere 30 miles, was not worth the risk. The parties should have just met up in person.
For COLLECTABLE coins/currency the insurance applies for Insured packages and priority mail as well. If it is just spendable coins/currency THEN it is only covered if sent Registered. I believe collectable cons/currency would also be covered by FedEx and UPS, but they have a $1K liability limit.