It seems like this should be the job of the FBI, DHS or some other agency like that. The Post Office should deliver mail and packages not investigate material on the internet.
Your workers are under threat from poorly made postal bombs that could easily blow up during processing, killing your workers. Due to the vast volume of post you process, the threat is real and non-trivial.
Do you do nothing?
I think you obviously have to do something, whether you go as far as they did is what's up for debate.