How safe is it to send cash by U.S. Priority Mail?
I have known a very small percentage of USPS employees who lost their job because of theft, so to minimize that scenario, don't make it look like there's any possibility of cash being inside. Sadly, one of the employees of which I speak, was rifling through obvious birthday cards for children.  Remember, this is not a common problem. In 30 years of postal employment, I probably knew 5000-10000 employees. Only 2 were thieves.If  you really want to send cash in the mail, use a security envelope that  can't be seen through. Of course the mail might be lost, or stolen by someone randomly pilfering mailboxes, but the chances are slim.  They're so slim, that my father once mailed me a $50 bill to Switzerland, where it was hand-forwarded it to me in Cairo, Egypt. Yes I got it, because it didn't look obvious.
THE POST OFFICE RIPPED OPEN MY MAIL AND STOLE A NECKLACE!?
First poster is funny... postal workers are federal employees. The "mayor" neither cares, nor has any power to change anything. Did you wrap the necklace, or was it just in an envelope? If the necklace was just stuffed in an envelope, it's most likely that it burst through the envelope on its own, due to being improperly packed/shipped. make another necklace. This time when you ship it, wrap it in bubble wrap and put it in a padded envelope. Wrap the whole envelope in packing take to reinforce it. I sincerely doubt some postal worker somewhere is wearing your homemade necklace.