AdMob in-app purchase allows you to set up ads to perform in-app purchases. While the SDK has added some APIs for this, you can't actually create in-app purchase campaigns in the front-end just yet.
The in-app purchase APIs offer two mechanisms to complete in-app purchases. One is the default purchase flow, the other is the custom purchase flow. The default purchase flow leverages Google Play in app billing for consumable purchases and the SDK handles most of the purchase flow for you. In the custom purchase flow, you have to perform the in-app purchase yourself. So with the custom purchase flow, you can use Google Play in app billing or some other mechanism.
Thanks,
Eric