The thing about communications privacy is that SCOTUS has ruled , correctly I think, that you have no reasonable expectation of privacy when you broadcast blindly into the airwaves or internet, it is a precedent that was set in the early days of car phones when they would often overpower CB radio's but seems to still be applied today.
As to Postal mail the courts have held that it is sacrosanct in order to protect postal workers and deter thefts, I really don't see any effective reason to expect privacy for some thing you leave in a tin box on the side of the road.
Where the 4th Amendment Should be all powerful is, as in your example, inside your home, "Secure in your person, property and papers" but that has been intentionally infringed, eroded and ignored based on precedents set in that raving success the "War on Drugs".
Do you know that if a "Drug raid" is conducted on your home all valuables, cash, cars, etc. can be seized under forfeiture laws now on the books ?
Do you realize there is no requirement to return them if you are found not guilty ?
In most cases it would be impossible anyway as they are auctioned, or otherwise disposed of long before you come to trial.
That is what several billion dollars a year is buying you, meanwhile the street price of cocaine and heroin continue to drop due to high supply.
But "It's for the children".
Bullshit.