Re: A letter from the US Postal Service??? How odd.
I ran into a very similar survey when I was tracking a package on USPS.com. Far as I can tell it was completely legit - no special offers, no request for detailed personal info, just some general generic info like age, gender, how often I use USPS.com and what I use it for, and no 'fall-out' after a month.
With what you have posted here, I would GUESS they are being legit about checking up on themselves.
I'm currently out of my mind, but feel free to leave a message, and if you would like to reach me by phone, please hang up now.