I've had one package that was coming from England that was seized by US Customs. I've had a couple I sent to OZ get snagged by their Customs. Even had one coming out of OZ that was seized.
I went through the steps of getting the permits from the USDA. It isn't very difficult and doesn't cost anything but time and effort. You need patience though as it took several months for them to send me the permits. Going the legal way also slows down the delivery process a bunch. I have to send the folks sending me seeds a permit, they're bright yellow stickers that are placed on the outside of the package, which are then mailed to US Customs. They can open the package or not and then forward it on to me if they're happy. You have to include postage and an envelope to get the seeds from Customs to yourself too.
In other words I'm a huge fan of birthday cards. The seeds have to be protected though, like POTAWIE said postal rollers are brutal.
Good luck.