From the information I have found, certain types of mail are irradiated (heat-treated) by USPS, namely post sent to government offices. However, I also understand that any mail considered suspicious may also be irradiated. This may have been discontinued altogether, I don't know. I'm just relying on statements made on various US government websites.
I just find it odd that I sent tens of envelopes of seed all over the world last season, and every recipient has advised of germination, except the 5 persons in the US who all had 0% germination. That would tend to mitigate against the squashing theory, as there should be less geographic bias.