As I talk to more US DG members and read some discussions, I've been told that some DG US members have trouble receiving seeds from Canadian traders. Most of the seeds that I have sent out got to their proper destination and were never flagged BUT I never had any CLAIMS FORM on it, just sent it in a normal envie as you would with a normal document. I also read somewhere that is it TECHNICALLY ILLEGAL for Canadians to send seeds to the US, but not for Americans to send seeds to Canadians.
My experience is that it makes a huge difference where your seeds enter the US. It might be more than possible that seeds from Canada enter via Chicago, which has the worst reputation when it comes to snatching seeds. News hasn't reached them yet that most seeds can be imported into the USA, if in small quantities. I suspect they just don't want to bother to look up which seeds can be imported and which not. Here you will find a list of seeds that can be imported (not only by nurseries but by "normal" people as well): http://www.aphis.usda.gov/import_export/plants/manuals/ports/downloads/nursery_stock.pdf
All seeds entering the US technically require a permit. If they are "small lots for gardening" and do not contain prohibited species, they do not require a phytosanitary certificate but the receiver should have a permit, which is free and valid for up to 3 years. All info, inclufing the application for the permit, can also be found on the APHIS Website.