So around noon yesterday, I called the number on the letter. I was informed it would be around a 15 to 30 minute wait. About an hour later, I was finally patched through to a human being. He sounded tired.
I explained the situation. He asked for the pertinent tax ID numbers and all of the name and address information. He looked things up and told me that everybody was calling in because the people who were processing the checks were very far behind. When they got around to my check, they would process it and then everything should go through properly and the interest and penalties would be cleared.
I thanked him. I felt sorry for him too, because it clearly wasn't *his* department that wasn't managing to deposit checks at any reasonable rate, but it *was* his problem to answer the phone for all the calls coming in from the people who had gotten the automatically generated letters.
Like me. :)