I received an email from Barnes & Noble informing me that I was eligible for another credit from Apple. The e-mail from Barnes & Noble stated “The credit results from the Settlements reached in an antitrust lawsuit filed by State Attorneys General and Class Plaintiffs about the price of electronic books.”
Curious, I googled for a credit from Amazon. I found this:

Apple eBooks Antitrust Settlement
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”). [snip]
Eligible customers do not need to do anything to receive these credits. If you are eligible, we have already calculated your credit and have added it to your Amazon account. As long as you have credit remaining, we will automatically apply it to your purchase of qualifying items through Amazon.com, an Amazon device or an Amazon app. [snip]
Eligible customers should have received a notification email from Amazon on Oct 18, 2017. This credit will expire at 11:59pm HST on April 20, 2018.
Your Settlement Credit:
Total Credit: $xx.xx
Unused Credit: $xx.xx

 
Go here to read about your credit.