In your settings, you'll find your mapped account that you chose to account the fee's against:
Sometimes a contact may require a different tax rate, so we can use this option to override the above tax rate with the one allocated to the contact.
To find this information in Xero, find the contact, click edit and head down to financial details.
You will find a section regarding the default sales tax. Note the wording may be different depending on your region.
The tax on sales is what will be used.
NOTE: If the invoice setting we are editing is set to 'No Tax' then an error will occur