@Marita11, it is good you reached out to AirBnB for guidance.
I don't know why the representative said your method was the recommended method. AirBnB's own page on Taxes specifically says:
"Some hosts are required by their local regulations to charge a tax. We recommend they include the tax in the price of the reservation, but some may require the tax to be paid directly upon check-in."
So unless the Hawaii tax is required to be paid directly upon check-in, AirBnB says to include it in the price of the reservation.
(And if the tax is to be paid upon check-in, you would not add the tax the way your are doing it now, which causes the guest to pay it months in advance.)
(Here in San Francisco the Hotel Tax is even higher: 14%. It is added automatically when the guest is shown the price of their trip, so the guest sees the final amount before they confirm their booking.)
Again, you are free to do things however you want. It is your property and your listing. And if you can convince AirBnB to give you an "Extenuating Circumstance" exemption, more power to you.
Just be aware that AirBnB might get tired of giving you exemption after exemption after exemption when it becomes obvious that the guest who protested about the price change is not that unusual.
There are plenty of places to stay on Maui where they tell you the final price up front.
I checked on over 30 random other AirBnB listings around Kihei. If the way you handle the tax is the "recommended" method, then almost no one else on Maui is following AirBnB's recommendation. Only one other listings handles the tax the way you do. Everyone else either specifically says they have included the tax in their nightly rate, or does not mention it at all.
We love staying on Maui! And we hope things work out well for you!
I want to help you avoid unhappy guests (that give you poor ratings) and unnecessary cancellations (that cost you money).