Marijuana seeds are legal to import IF they are sterile (unable to germinate) which does noone any good. Customs, due to the confusion on the subject, will confiscate all seeds whether sterile or not if they are found.
So in answer to your question, No, they are not legal to import.
Coca leaves gets a hell no and you won't just get a letter if you try importing it. You will get a friendly visit from your local DEA.