By the time you become a driver you have spent at least 5 years with the company, so if there were any doubts about your integrity, it would normally have surfaced before you then while you were a part time loader or unloader. Each driver had also written down addresses of the 4-5 pkgs we took, so if there were any questions, the company knew right where to go, and we used paper records to get signatures.
Most of the time they were packages for your customers on your normal route that had arrived after you left the building in the morning, so these were your friends you would be stealing from, so just not worth it. No package was worth losing a good salary and benefits for what you may think is in the box. I worked for them for ten years and never witnessed or heard of a driver stealing, ever. I hope that answers your question.