Amazon Mobile Ads API
developer.amazon.com
developer.amazon.com
2 important points from the FAQ [1]:
Q: Are apps that use the Amazon Mobile Ads API required to be distributed through the Amazon Mobile App Distribution Program?
A: Yes. Apps that use the Amazon Mobile Ads API must be distributed through the Amazon Mobile App Distribution Program before they can be distributed through other Android platforms.
Q: Which regions are supported?
A: The Amazon Mobile Ad Network serves ads to U.S. users.
Also it apparently doesn't play well with AdMob mediation. You have to use custom code to mediate between Amazon and other ad SDKs [2]. No info about which mediator SDKs are currently supported.
[1]: https://developer.amazon.com/sdk/mobileads/faq.html [2]: https://developer.amazon.com/sdk/mobileads/with-other-sdks.h...
Even if you can get good, independent mediation with per-service, per-country settings and good backfill - which itself can be difficult - it can take a couple of seconds for that second ad backfill to come in, which usually means a dropoff in clicks.
Whenever I see second tier ad services boasting good eCPM, I always wonder what their fill rate is, especially if you're app isn't something you'd expect lots of ads for, like information on cars for people in the USA.