Several users have asked how a the total sales tax charged can be calculated and aggregated for multiple orders (invoices). This information isn't separated in the 'Reports' nor explicitly stored in the 'Orders' sheet; however it can be derived from the invoice information that is saved in the 'Orders' sheet.
To calculate the Sales Tax attributable to the first order (saved invoice), the following formula must be entered into cell M2 of the 'Orders' sheet.