Did you know that In-app purchase feature can cost you money on the Kindle Fire? If you don't want to spend money more than what you have intended for the apps, then you should disable the In-app purchase function.
In-app purchase are generally use to allow you to play more levels in the game or you will have to pay a subscription to play the game further. Some of the free games that you have downloaded use this tatics to make money. They let you play for free for the first 3 levels and then once they got you interested, you will have to pay to play more levels.
This feature is enable by default, so here's how to disable it:
1. Tap on the Home key to return back to the Home screen.
2. Then tap on Apps.
3. Select Store.
4. Now tap on the Menu icon .
5. Select Settings.
6. Then tap on In-App Purchasing.
7. Now configure to have it disable or enable.
At a later point, if you decided you want to make a purchase of a subscription, you can come back here to enable this feature again.