I don't think a dog can pick up the smell of seeds. So when you get the seeds, you write in big letters "return to sender" and let it sit out in your house somewhere for a few days. If you get a little visit and are absent-minded enough to let the cops in, the package is sitting right there, ready to go back to the post office. "Oh, sorry, officer! I got this odd little package, and I have been meaning to bring it back to the post office, but I've been too busy! There must've been some kind of mistake."