Just a quick reminder for anyone that uses the Amazon Appstore: You can get a small amount of coins for free each day.

Where to find it in the Amazon Appstore (imgur)

It's not much at about 2-6 free coins every day, but I've gotten up to 24 before. You might get yourself a free Fountain of Lapis eventually.

Note: May not be available in all countries.

Edit 1: u/kryfton says he's received up to 100 coins from the daily before.

Edit 2: u/GaymerGuy79 received 500 coins once and states that you can see your daily coin received amount in your history on Amazon. Link to comment below.

Edit 3: If you're outside of the US and don't see the gift box, u/ec0188 has a tip:

I'm not in US, you may try to change your Amazon account country setting to US and then give a try.​ Your Account > Digital content and devices > Content and Devices > Manage Your Content and Devices (Preferences) > Country/Region Settings. Link to comment below.

Edit 4: u/Mikumarii confirms below that you do NOT need to be a Prime member to see the daily gift box.