As I moved from Flash (Adobe Air) to OpenFL, the same I’m doing with extensions.
First extension, Admob OpenFL Extension could be installed with haxelib:

haxelib install admob-openfl

Full source code and usage instructions could be found on GitHub:
Admob OpenFL Extension

Unfortunately, extension supports only Android, so if anyone wants to add iOS support, please let me know!

з.ы.: Amazon Ads extension is coming soon… 🙂