My understanding is that shipping anything in the USPS system that is derived from the cannabis plant is Federally illegal. Seeds are a part of the plant. It doesn't matter what your intent is after buying them. It's the act of sticking them in the mail system that is definitely illegal.
With that said...in my 20+ years of ordering seeds I can only remember one instance where someone was taken down by confiscated mail. It was a good decade ago if not more...in Cali...and the recipients were operating a large hash oil manufacturing facility. the DEA was already on their trail...and the seed shipment was their easy way in.
Either way...it's never good when you get notified by the FED that something is amiss. Definitely rattles the cage.