Yesterday I received an email that contained a promotional code for $5 worth of credit in the Amazon Appstore. I didn't sign up for anything special but I do receive their appstore email newsletter regularly. There are all kinds of neat apps available and with the credit would be free. The title of the email is "Complimentary $5 Credit from the Amazon Appstore" Below is a bit of what it looks like.
**Also please note that the codes look unique so if you want to use it don't share it.