If an enemy wanted to ruin our agriculture, why wouldn’t they mix the evil plants in a hydroseeding solution and drive around the countryside spraying it all over ag fields? They could cover thousands of acres with noxious weeds in no time at all. Sending mysterious, unknown and unordered seeds seems like a stupid thing to do with almost no chance of success.
Because they can't get them past US Customs. Why no one is checking USPS packages at the border is the mystery.