This offer for those with a Kindle Fire with Special Offers only. On your Kindle Fire with Special Offers, swipe the top menu to the left until you see Offers, then press/click in order to see the current offers. At the bottom, you should see the offer to 30% Off Select Kindle Accessories. Click on the offer, then on the orange button to add the credit directly to your account. It's nearly instant and the next screen offers to take you to Amazon so you can spend that credit away! If you miss that screen, though, don't worry - a document about the offer will appear on your home page.
The discount is limited to Select Kindle Accessories and they must be "sold by Amazon.com". You can put multiple items into your cart, then go thru the full checkout process - you should see the discount in your Order Summary at the upper right corner of the page. The total discount is limited to $30 (30% off $100 in purchases) and will expire at midnight (PT), October 15.
There are the usual assortment of cases to choose from (although, not the ones actually made by Amazon), along with stands, earbuds and styluses that will work with many different devices.