If made correctly, they go off when you open the package, which isn't done in the post office or the truck.

Package bombs are designed to go off when the box is opened, usually with a spring-loaded system. But, you are right that it's not the most foolproof in ensuring your goods reach the intended destination. That's why you really don't see it too often, with people preferring the bioterrorism route if they're doing terror-by-mail. Especially after the anthrax attacks in 2001.

It's worth noting that The Unabomber was a genius level intellect and a math professor who had a pretty in-depth knowledge of mechanical sysyems. He's kinda the main case, in the latter 20th century, of someone actually pulling mail bombs off successfully.

There actually were 2 separate mail bombers in 2018, that I know of anyway. One had a bomb go off in a FedEx facility, so that answers your question there.

The other bomber had every single bomb reach its target - people like Obama, Kamala Harris, George Soros, and Robert De Niro. He failed because none of his bombs had a trigger device.

So it seems that, in the history of mail bombs, you either are a genius, you forget bombs need to explode, or they blow up in transit.

The mail carrier doesn't light the wick until its delivered.

Talking out my ass, but not all explosives are vulnerable to jostling. I.e., a solid state excitable may require a high energy trigger. Like how bullets don't explode whenever. Also, packing peanuts.

The bomb doesn't go off until the box is opened.

Short video one shows how a hand grenade works.

Short video two explains that putting a grenade in jar (a glass box) will stop it from detonating until the glass jar breaks (the box is "opened")

The box, be it a glass jar, or a cardboard box, stops the bomb from going off until the box is opened.

• DOJ Office of Inspector General report reviewing FBI Laboratory forensic examinations of all 14 Unabomber devices, including critiques of explosives residue analysis methods: https://oig.justice.gov/sites/default/files/archive/special/9704a/17unabom.htm
• FBI Turchie Affidavit containing detailed forensic descriptions of all 16 Unabomber bombs, including component lists and trigger mechanism analyses from court records: https://www.thetedkarchive.com/library/bomb-parts-descriptions
• National Archives blog post about the lead prosecutor’s files, describing how the UNABOM Task Force organized forensic analysis across FBI, ATF, and Postal Inspection Service: https://text-message.blogs.archives.gov/2023/06/06/insight-into-the-unabomber-case-from-the-files-of-its-lead-prosecutor/
• DOJ standards document for forensic explosives and hazardous devices examination, covering IED component segregation and analysis procedures: https://www.justice.gov/archives/dag/file/877721/download
• National Institute of Justice bomb scene investigation guidelines developed by the Technical Working Group for Bombing Scene Investigation: https://www.ojp.gov/pdffiles1/nij/181869.pdf
• FBI’s Terrorist Explosive Device Analytical Center (TEDAC) overview, describing how they analyze IED components and maintain a national database of device designs: https://www.fbi.gov/investigate/terrorism/tedac
• UN Mine Action Service IED Lexicon defining victim-operated IEDs (VOIEDs) and the three main categories of IED switches (command, time, victim-operated): https://unmas.org/sites/default/files/unmas_ied_lexicon_0.pdf
• Wikipedia article on the 2018 Cesar Sayoc mail bombing attempts, noting FBI found the devices lacked proper triggering mechanisms: https://en.wikipedia.org/wiki/2018_United_States_mail_bombing_attempts
• AOAV (Action on Armed Violence) technical review of IED types and switching mechanism developments: https://aoav.org.uk/2016/ieds-review-different-types-ieds-exist-focus-recent-developments-ied-usage/
• Forensic Science Simplified guide to explosives analysis explaining how bomb squads and forensic labs examine IED components: https://www.forensicsciencesimplified.org/explosives/how.html​​​​​​​​​​​​​​​​

2925 version: Because you don’t push the software update meant to fix the exploding issue until after delivery.