Get insurance and lie about the value. There would be no way for them to prove otherwise. Make it more worth your while.

And, honestly, the instances of the USPS losing stuff is relatively small. They just get bad press because it's a government run institution instead of a private org. Anytime something goes even minutely haywire, they get lunged at. UPS or FedEx goes awry, people brush it off. I think the perception of the mail system would be different if they were privatized like UPS and FedEx. Alas...