Back when I was working in shipping, there was astonishingly little trace of thievery anywhere. It was known that it had happened but it always resulted in immediate termination of employment and criminal charges brought up.
Oversight, cameras and security measures were all there, but it felt like the biggest thing keeping stuff from not being stolen was that everybody was so busy and absolutely fed up with whatever went through their hands. Sure, if somebody sent a really valuable item you'd notice it and comment and joke with work mates but that was about it. "Hey did ya see that 10 000 shotgun that the previous guy sent?" "Yep, amazing what some people will pay for their stuff. It's a strange world. What next?"
The bigger concern was that some outsider (passers-by, dishonest customers) could pinch stuff from shelves if not supervised. The occasional junky or habitual thief would sometimes grab stuff and run off but that's a risk with any business and that's why there's insurance.
Things were lost sometimes when packaging failed but even in those cases, most were recovered. Then there would be emails telling "we have this funny bit here with no info, looks like such and such, anybody missing one?" Most found their owner.
So yes, postal and shipping employees can steal things, but in my experience it is very rare and in most cases the stupid mofos will be caught and prosecuted.