With Sunrise, you can create or customize invoice templates to reflect what you call your taxes.
Head to Settings > Templates and either Edit the default or create a new template for your new tax name.
For Canadians, there is just one more modification needed…that’s to rename your Invoices to “Tax Invoice”. Go to the template, select the ‘Language’ setting, and then scroll down till it says ‘Invoices’. Under that
Go to the template, select the ‘Language’ setting, and then scroll down till it says ‘Invoices’. Under that there are email subject/message boxes, then under that, there’s “alternative document title’. Change that to ‘Tax Invoice’ and voila!