I worked for a jewelry store for 10 years. When we needed to send a customers jewelry some where else we mailed it (USPS) Registered mail, return receipt requested. It costs a little more but everyone who touches that package has to sign for it so there is a direct trail. In 10 years I never had a single item lost.
I try not to mail anything with sentimental value (all my quilts for example) but if necessary that's the way to go in my humble opinion.
Jo