From Amazon:
In November 2014, a federal court approved a Settlement of antitrust lawsuits brought against Apple, Inc. (“Apple”) by State Attorneys General and Class Plaintiffs about the price of electronic books (“eBooks”). Those settlements resulted in credits for qualifying Kindle books purchased between April 1, 2010 and May 21, 2012. These credits are funded by Apple.
Head on over to Amazon to see if you’ve got any credit.