Earlier quoted context omitted.
I'll argue that that should work, on some level. Communication with one's attorney is privileged for a good reason, and under your theory the police can interfere with that whenever they imagine the package you've mailed isn't a communication.
I failed to see how you conclude mailing a bloody knife should work from your second sentence (which I agree). Communication should/is privileged, but obviously not any package should be considered as "communication", and even at the worst scenario, having the police to interfere the package whenever they want still doesn't stop you to practice your privilege of communication since there are lots of other forms of co…
I don't think this is a cut and dried case where it's obvious the authorities were in the wrong, but the above doesn't really pass muster with me. Where is the line deciding which "communications" are and aren't eligible for attorney-client privilege? For instance, if the package contained printed documents, would that be protected?