I'm about to receive my first shipment of seeds, to Italy.
So I don't know about seeds, but I bought and still buy a lot of things in US.
Italian customs does something different everytime, so even if I try to be prepared, surprise is always around the corner.
Once I got a box full of candle scent samples, it was a gift from a friend.
The man at customs called me and asked me what was that, I explained to him and he was so amused, he sent the box right away.
Another time I bought some bar of soap, and customs needed me to fill a questionnaire about the nature of those, it took some weeks then I got my box, paying for that.
I guess small packets and envelopes just slip and pass, they don't check everything.
Sometimes me and my friend choose to cheat a bit and write something "innocent" on the box or on the envelope. And we use to write "gift" on it, and declare a very low value so nothing should be asked.