This showed up on my inbox tonight!
Recently, you used a portion of an Amazon.com promotional certificate when you ordered with us. Due to a processing error, we did not make the remainder available to you for future use, so we are sending you a replacement certificate instead, included with this e-mail.
And it's a legit!! I think it's from the MCI $150 deal. I purchased a few items then cancled the order. The order balance was never credited to my account. I called and complained, and an 'order specialist' took $45 off another order of mine. But I guess Amazon's system still showed that I was owed $50 (which was the approximate amount of my canceled order). WOOO HOOO!!!