Actually... you can do more. You have a web form. Process the results with some error checking. I'm pretty sure Google (maybe someone else) has an API that lets you pass them an address and they will validate it. So you could pretty easily throw an error for forms that do not have a complete address. That is not legal advice... that is "Form Processing 101".

Having said that, I think it is rather comical you are using old school snail mail to request the NSA to provide people with their digital records. Plus I'm sure the paper form is a bit more work for someone to process. I like it. I'm all about using inconvenience as a middle finger.