Both replies are correct - to be legal, you must have a USDA permit, and a copy of the permit must be enclosed in the package, along with a list of the seeds and where they were collected. Additionally, there is a little green and yellow sticker that goes on the front of the package, and that sends the package to the USDA. Then they forward it on to you after they make sure there are no "nasty beasties" inside. The permit is free. When I order seeds, I email a copy of my permit, seed list for them to fill out, and a color copy of the green package label, and ask the seller to fill out the list (sometimes I do it ahead of time, to make sure botanical names are correct and readable, since that can hold up a delivery) enclose the list and permit in the package, and print a COLOR copy of the label and attach it to the package. The only thing that ever goes wrong is that they don't have access to a color printer...
On the other hand, it is pretty common to just order seeds and have them shipped over. I would guess the odds of the package not getting seized is maybe 50-50, or maybe even better. So I guess it depends on how much the seeds cost and how big the fine is, if there is one, as to whether you want to risk it.
Carolyn