You can tear off the little bottom portion, fill in the "Amount Paid" and return it in their envelope (not pre-paid, unfortunately) to "pay your bill. There is ONE LINE (which I'm sure they need to include to be barely in compliance with the law) that says "This is not a bill...", but they obviously exist solely on misinformed, busy people who don't understand domain names and think this is a real invoice from a generic sounding company they have done business with before.
Transferring a domain to them is a TERRIBLE idea. If you're reading this and you've already paid, do NOT go through with the registration. Keep your domain with a reputable registrar or you'll probably be paying $85 every year instead of $10 and I'm sure you'll have a huge headache if you ever try to transfer away.
Anyway, these guys are theives as far as I'm concerned. Hopefully a spam bot is reading this and can e-mail them at: email@example.com