How does Amazon app store work?

Over Black Friday, I bought a Kindle Fire HDX. Nice little device to along an Amazon prime membership.

Yesterday, I was setting up the device to match what I use on my iPad. Part of this involved buying Instapaper so that I can get my readings done.

Here is how the process went.

On activating the device, Amazon sent me 500 Amaozn Coins. This can be used for App store (and others) purchases.

I saw Instapaper costing $2.99

So I spent 299 + 25 (tax) = 324 coins when I purchased it.

This triggered a promotion and returned 250 + 90 = 340 coins to me.

So I actually made 16 coins on this transaction!

I wonder how much is the payout to the author.