Thanks, I was able to view the document in the first of the two links.
I don't doubt the veracity of the letter, but I do question whether it was truly the Postal Inspection Service which made a complaint. I worked for the USPS for almost twenty years, and they don't use the name Federal PO Inspector, nor do they use the acronym FPOI. The Inspection Service doesn't get involved much in routine complaints, they have bigger fish to fry; mail fraud, theft of the mails, etc. I think it more likely that a local supervisor had received complaints and made a call to the elder.