I agree with the others, this sounds like a phishing attempt to me. I got a post card in the mail like enolaeagle mentioned just a few weeks ago. Same thing, fill out, mail in. It never ceases to amaze me the extent these scumbags will go to in order to get information from you or scam you.
For kicks & giggles, have you tried to google the website they direct you to in order to see whether or not others have done so and had issues? Just be careful that your google link doesn't direct you to the website in question. Look for some sort of forum or who is this page. I know when I get these spam phone calls, despite my registering in the National Do Not Call List, I google the number and there are pages upon pages of folks complaining about them.
Sounds like your situation is very similar to this except that they're sending you a physical letter.
You’ve Got Mail: USPS Phishing Scheme
My local post office is great, I've nothing but great things to say about them. I try and use USPS whenever possible for shipping.
Fred, do let us know what they have to say when you take the letter to the PO.