Last year, Apple was ordered to refund ebook readers who purchased certain books between April 1st, 2010 and May 21st, 2012. The company paid out $400 million to impacted consumers who should have seen credits added to their various accounts, on Amazon, Barnes & Noble, Kobo, and even Apple's own iTunes. If you received Amazon credit, you have until 11:59PM PT on June 24th to use it.
You can check your credit here. You probably used up all or most of your credits, so long as you’re still buying Kindle books because they’re automatically applied to any qualifying item. Plus, you also should have received an email about these credits when they were issued last June.
Anyway, if you somehow haven’t bought enough ebooks to use all your credits, get to it. You have five days.