Booking.com taxes are primarily controlled in Booking.com. If you already have an established Booking.com listing and are connecting it to a Hostfully property, we do not override tax settings on your listing. Make sure to set up your taxes in Booking.com and if you still want to send additional taxes/fees you can set those up in the fees and policies tab in Hostfully.
The only time we send the value in the field "tax rate" to Booking.com is if you are creating a Booking.com listing from a property in Hostfully. Then during the build process the tax in the Hostfully property is pushed to BDC, but after that all tax changes need to be requested in Booking.com Extranet. When a listing is built, the default setting is
net price +
locality charges +
property charges (screenshot below on Booking.com Extranet)