The Amazon Appstore Android App received an update to version 9 earlier this month that introduced carrier billing. This feature will enable some users to add app purchases to their mobile bill at the end of the month rather than supply Amazon with a credit or debit card. The option is only available in select countries.
Unfortunately, that's all we know about the feature. Its addition doesn't come as a surprise in and of itself, given the company's movements in that direction over the past few years, but we haven't come across any details.