I'm going to try to stop crying long enough to share some more detail in case it can help others think about what they might do, or questions they may have, if faced with a similar situation.
I live in Maine, and as noted in another thread, the state laws here are particularly strict. No freshwater snail has ever been approved for sale in the state. There's a reverse burden of proof here, and unless a species of any pet has been specifically approved for sale in petstores, it is not allowed. In LFSs, there are no shrimp, no African dwarf frogs, no white cloud mountain minnows, no brigs, etc... Many things readily available elsewhere are not approved for sale here.
However, I think that the USDA is only concerned with federal law, and so I don't think that Maine's strict stance had very much impact on this particular situation. I couldn't, though, claim that I got these at a local flea market and that they had not crossed state lines. If I had them, my guilt there was obvious.
The shipment they had record of included more than the two marisas. There was also a cana, spixis and cherry shrimp, none of which should be in Maine according to state law. I told the rep from the USDA that the cana died. He made note of this. I did not have to produce a shell or answer any more questions about the cana.
He did not ask me any questions about the spixis or cherry shrimp. He did not ask me if I had received any other shipments he should be aware of.
He brought me the section of the Code of Federal regulations that gave him authority. (I'm sorry if this all sounds cold! I don't mean for it to, but I also think I'm now paranoid and don't want to say anything here that I could get in trouble for.)
It says-
The Channeled Apple Snail and the Columbian Snail are classified as plant pests, as per 7 CFR 330.200. No person shall knowingly move any plant pest... interstate, or knowingly accept delivery of any plant pest so moving unless such movement is authorized under permit... The movement of snails and slugs, as well as other plant pests, is governed by such provisions.
In the ID of pests section of the form, he listed:
Marisa cornuarietis
Pomacea canliculata
I had to sign and date a form. I'm quite sure I would have had a visit from him and would have had to sign this paper even if all of the snails had died.
Again, he asked no questions about the rest of the critters in the shipment.
I'm freaking out because he brought me a full package of educational materials on invasive species including magnets and stickers. He was a very young man who seemed like a decent person who was visibly moved by the fact that I couldn't stop crying.
But yes, those two snails are gone.
I sincerely hope this info can be helpul to others.
Very sad Clam