Why aren't post office staff prosecuted for stealing mail?

I sent my grandson in New Zealand some dollars for his birthday. The package was very securely sealed. It never arrived. I have sent numerous packages to him and he has always received them. Post Office staff would only know what was in the package by scanning it. Obviously then the staff who scanned it saw the dollars and stole them. When I inquired I was told that I should have payed a fee to have it delivered by registered mail. Isn't that extortion? You have to pay so as NOT to get your stuff stolen! An employee of the Post Office scanning department is a criminal thief!