I’ve received word that Amazon is giving bonus Amazon Coins for purchasing any of these apps. It is unclear right now the exact details of this promo, but it looks like purchasing any $0.99 app will result in a net gain of coins. I personally purchased this Solitaire game for 99 coins and received 159 coins back. A net gain of 60 coins. Let us know in the comments which app you purchase and how many coins you get back.
Like with so many of these free coin deals, this one is very hit or miss. Some people are getting bonus coins and making a profit while others are only getting the coin amounts listed on the app page. The coin reward amount seems to vary from account to account and there is no way to know ahead of time if you’ll get bonus coins, so please proceed with the assumption that you may not get any bonus coins.