Sales tax is already there and available; it is a Function, not a field or a custom field, in that is built in and attached to the Sales Tax Functionality. You would turn on sales tax management in the file, set up the Rates, assign which customers are taxable at which agency or combined rates, assign a default taxable or not to each specific items, and then the program will compute and assess, based on what gets sold to whom. You see all of this on your screen and in the forms.