This letter is from the Internet Domain Name Services. You can get a full-sized PDF of the letter to follow along.
One would think it was illegal for these companies to send out these letters. But really, the language is carefully crafted to skirt laws. Even though right at the top, in large type it says, "Domaln Name Expiration Notice", in the body of the letter, they say, "This notice is not a bill, it is rather an easy means of payment should you decide to switch your domain name registration to Internet Domain Name Services."
What makes this a scam aside from the deceptive language is that you receive no advantage from transferring your domain registration to these services. Instead you will end up paying a great deal more than you do from most registrars.
So what should you do?
Throw the Letter in the Trash!