We can't automate the use of sales tax for some charges and not others. Some users of Enrollsy have a value-added tax that applies to all charges/payments.
That's probably not what you're thinking about, so instead of trying to automate the application of tax, head over to My Company and choose the Items tab. In there, add a new Charge Item called Tax and save it.
If there is an item that is most commonly sold, and you have the sales tax calculated, you could add that amount as the default tax amount and override it when needed.
Alternatively, you can leave the amount set to $0, and you will be able to add the tax to any transaction and simply enter the calculated tax amount for each item you sell.