How can I get the Return Receipt Signature from my Certified Letter?

As of May 31, 2015, USPS has discontinued this product (Return Receipt After Mailing PS Form 3811-A). If you think you MIGHT need a Signature, you're better off purchasing the Return Receipt at the time of mailing (when you create your label), because USPS will no longer sell you the Signature after the letter has been delivered.