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.