Amazon recently announced that they are no longer going to allow people to buy audiobooks with the Audible App and ebooks with the Kindle app from their apps on Google Play. This is because Google recently implemented a policy that all in-app transactions are handled through the Google Billing system, whereas before companies had their own billing systems.
Read the original article here: https://goodereader.com/blog/electronic-readers/here-is-how-you-get-audiobook-and-kindle-to-work-on-e-readers-phones-and-tablets