To me these two tags seem to refer to the same idea. If I make myself stretch to find a difference between them it would be that perhaps in-app-billing could refer to an ongoing payment like a subscription, and in-app-purchase may be intended for a single purchase. They don't seem to have been used in this manner though.
The main distinguishing fact in the questions tagged with each respectively is that in-app-purchase seems to be used more on iOS questions, and in-app-billing seems to be used more on Android questions. However many questions use both.
If these tags should not be merged would it be appropriate to edit their wiki's to make reference to each of the mobile platforms respectively? As it is now they both seem to cover the same territory.