Just makes you wonder how alert the owner, and the postal people, really were.

Did the package not feel heavier?

Did the cat make absolutely no sounds or movements or anything while you taped, carried and then jostled the package around at the post office?

Did the poor cat not cry while in the back rooms or the delivery vehicle??

The scenario just seems so odd. I could see a cat going for a ride in the back of a truck or something like that, but a package that you handle... hrm. Even my lightest weight cats, and my not very talkative ones, I think I'd notice and they'd put up a fuss.