I live in US, and have always ordered seeds online, usually from Attitude Seed Bank, which I'm pretty sure is headquartered in Amsterdam. Have made 4 or 5 big orders over the years... no real issues.
Since marijuana seeds in and of themselves are not illegal to own/posses, even in non legal states, you'll have no issues.
I always buy the "discreet" packaging when I order (which usually means it comes wrapped in a T-shirt, not in the original packaging). I once had my package arrive, it had been opened, and had a pink paper taped to it, saying that it was opened at the customs center in NY... my shirt was there, my seeds were there, and I had no issue then, nor ever.
Not sure if that makes the prospect of buying online MORE sketchy.. but in my mind, it proves that even if the absolute WORST happens (customs finding your shipment), nothing terrible will come of it, because again, the seeds arent illegal to own, only to grow (and only in the 18 stats where marijuana isnt even partially legal).
With marijuana being fairly mainstream, and legal in some form or another across majority of the country, I wouldnt concern myself too much with the purchasing of the seeds.
If your paranoia still has the best of ya, go ahead and find a US based seed bank, so the package isnt going through customs.. and I'm sure others can recommend US seed banks/strains.
Hope this helps!