Amazon always offers up to a 10% discount if you buy Coins in bulk, but for a limited time the 5,000 ($50) and 10,000 ($100) lots are available at a 20% discount. This drops the prices down to $40 and $80.
These Coins are a form of virtual currency that can be used to purchase apps and games at the Amazon Appstore for Android. Remember, Coins can be used for more than just Kindle apps/games:
Now Android phone and tablet users can save up to 10% on apps, games, and in-app items with Amazon Coins. Coins are only available on the latest version of the Amazon Appstore.