I had the pleasure of working for the IRS many moons ago. I was a Remittance Perfection Clerk. Basically, anything that had a letter attached, or a check for a filing without a signature, wrong amounts, mixing up personal w/ business returns etc. was sorted and forwarded accordingly by us. I got to see some really interesting shit there and notices for less money than a postage stamp like this and the resulting angry correspondence were all too common.