Okay... since you're also taking the day to reply to every single post (and bump your thread), many varieties of fungi have long-lived spores. Non-native species have a nasty habit of coming into the country and causing problems with the ecosystem. Even foreign versions of local plants are dangerous, if not more so since they can cross-breed and create hazardous new hybrids. Chytrid Fungus is a good example of a problem fungus, though not directly related to your ornamental mushrooms.
The particular species you are discussing is infamous for its resurrection, and is also considered toxic unless steps are taken to remedy that. Was your batch dried and "dead" to the point that it was harmless? Was it packed properly? Were they rendered non-toxic by some documented process?
Outdated, but relevant:
So you would not need a permit to import mushrooms for food consumption. Unfortunately, you have told us these are decorative/ornamental. You do not need a permit as of that publication to import dried mushrooms, so long as they are free of soil, insects, disease, and contamination. You said, however, you filled out the appropriate paperwork. This would seem to suggest that either you misspoke, or your mushrooms would not have fallen into this category, either.
In any event, you have not provided (and please don't at this point) the reasons given for confiscating the mushrooms. This is why Amy, myself, and a couple of others, would assume the most likely scenario, which is that your mushrooms were cross-contaminated in some way or were not sufficiently rendered inert by your packaging and shipping processes.
The suggestions to contact lawyers and insurance companies are sound. I am surprised you did not have a company insuring imports like this against damage, loss, theft, or contamination.