Having been in the Kindle and Google Play (and iOS) app stores for years, I had previously written a library that wrapped Android purchasing so my apps could share one code base. However, that library became outdated and then obsoleted as Google forced people to update to the new billing library. I stopped updating my Kindle apps as a result several years ago.
Now, I’ve got around to updating them and found the (new?) Billing Compatibility SDK and I have to say that overall it has been great! My Google billing code that was shiny and fresh from a complete rewrite was /mostly/ able to convert directly. And, with a bit of tweaking with Gradle flavors, I don’t need to touch the code to recompile either version (documented here at Stack Overflow).
Overall this was better than I’d hoped. I did go on a long tangent trying to get subscriptions to work because I missed the part about copying the (unnamed) amazon.sdktester.json file into the assets folder but once I realised and corrected that, I was pleasantly surprised that it all “just worked like Google”.
Kudos to everyone involved!