But that's mostly because it's impractical. They do use dogs to sniff for drugs and explosives, so if CSAM smelled or was visible through X-ray then it would probably be a different story. And let's not forget snail mail is by far a more uncommon way to spread that material than the Internet is. The Internet came into broad use just ~15 years after commercial CSAM was openly being sold by mail order in Europe.

Personally, I am on the side of privacy, just to be clear.

If East Germany can, why can't we? /s