If you are using the Amazon appstore ecosystem, Amazon has launched a new promotion where you earn Amazon Coins when you purchase select apps. The promotion includes 45 camera and photography apps including PicShop - Photo Editor ($2.99, earn 90 coins) and Photo Editor+ (99c, earn 30 coins) and XN Retro Pro ($1.32, earn 40 coins). Coins (1 coin = 1 penny) can be used to purchase apps at the Amazon app store.
TIP: you also earn coins when you get eligible free ($0.00) apps. The individual pages for the eligible apps at the earn Amazon Coins page do not show the amount, but you do earn coins when you get the participating free apps as well. It takes a few minutes for the coins to be added to your total, but they do. Then you can use your newly earned coins to stock up on camera and photography apps!