It really depends. AirBnB *does* charge the guest (up to) 12% across the board. THEN In some jourisdictions guests pay Occupancy Taxes, in some they pay VAT, etc. What you should treat your room rate as is: "Suggested retail." They then pay any local, regional, state and federal taxes on that retail price, they then pay the surcharge by AirBnB for that room.
You just need to refer them to their receipt. That you have nothing to do with billing except for adding additional items per the guest's request.