United States Postal Service U. S. Postal Service employees stole items from an envelope my sister mailed to me chino hills, California

My sister mailed me a Christmas card from El Centro, CA, put in the box at the post office , with a $50.00 gift card,  a picture, and some perfume samples.  When it arrived at my Chino Hills, CA home, it was in a plastic bag with an apology from the Post Office for the condition of the envelope.  The envelope looked like it had been cut open, and then re-taped.  The only thing left was the Christmas card and one perfume sample.  

My sister already talked to the mailman who told her unfortunately, since she didn't insure the envelope there is nothing we can do.  

I guess I just want people to know the U. S. Postal Service is full of thieves, as the envelope was in their possession the entire time.