By federal law, its illegal to possess seeds.
Meanwhile, customs job is not to let the contraband into the country.
So if customs finds any, they're not going to send the seeds to you; they confiscate them!
Meanwhile, if they're confiscated, you don't "possess" them, do you?
Anyway, I can definitely see why you might be paranoid about having seeds sent to your home, but in practice, I never heard of anyone actually being busted over this.
Customs policy, as above, is that if they identify seeds, they confiscate them and send a note to the recipient saying so.
They simply don't have the time, resources, or inclination to open up a Federal drugs case over a few cannabis seeds, especially when they've got literally hundreds of thousands of packages and letters to go through, looking for actual drugs, weapons, and other contraband.
If customs doesn't snag the seeds, there is basically no chance anyone else will down the road from them. Its against Federal law to open up sealed mail, and even law enforcement can't legally do it without a search warrant.