this is just coming off the top of my head and maybe not the best idea haha but you put a bomb the package with the weed and if sumeone from the postal service checks it it blows up because they do not have the special deactivation system to disarm it which the person ur sending too will have....or you could go with the peanut butter idea because that sounds more realistic.

but another thing you could do is send it in a bag of chips, like open the seal then reglue it shut after.