Interesting subject. I have received several packages from my vapor store and the last two boxes (USPS Priority) were opened. I really believe it is the adhesive on the box. I have received many packages for various items from China and they were never opened.

Have never had to sign for anything. I don't think the usps is opening anything on those priority packages. I use them myself and always tape up the ends because the adhesive just isn't very strong.