We tend to sometimes overlook the technicalities, after all a small packet of seed in a letter would be hard to find BUT the facts are the facts.
I spent a week going from phone referral to phone referral to answer this question. The answer hasn't changed.
To receive or send ANY seed or plants including TC'd material, requires a permit and phytosanitary certificate *bare minimum*, and possibly other permits from the FDA and Fish and Wildlife management. Inquiry must be made mentioning specific species, and they will let you know what you need.
Most likely, the world will turn as it will, and seed will be sent and received as it always has been, but you should be aware that it is not legal without the proper paperwork, and there are possible fines if the material is intercepted.
Personally, I have never had any problems with customs getting or sending seed, but the potential exsists