Absolutely NOT a revenue use; there was no one cent rate. This was a drop letter. I happen to know the seller and this was an honest mistake. It's nice, but I don't know why anyone would go this crazy for it.
That realized price is insane. There must be something unique about the letter itself or the scarcity of a White Pigeon, MI usage. Either that or someone thinking it was a real revenue usage, but even then it's at most a $250 item depending on date and rate, with the majority falling in the $100-200 range.
You will frequently see postage stamps affixed to insurance policies claimed to be revenue usages but they are not. Instead, like this document, it was for the postage to send the document through the mail.
Insurance policies with both revenue stamps and postage stamps affixed are in-demand collectibles, usually ranging from $50 to $200 depending on the venue/dealer.