I have a couple of questions regarding sales tax.

1. What is the correct method for calculating sales tax for a US based business/website - by entire order total or by individual line item (i.e. order subtotal * tax rate versus sum(line item * tax rate))? From everything I've seen, it's by order total, not by line. Can anyone confirm?

2. If you're calculating sales tax based on order total, when issuing a return do you simply recalculate tax based on the remaining order total, after subtracting the return, and then take original tax - new tax and refund the different along with the line item(s) being returned?

3. If a sales tax rate is changed for a given area/zip code and a return is being issued, would you refund the customer based on the sales tax rate of when the order was purchased, or the sales tax rate that is currently in effect at the time of the return? (I'm assuming it would be based on the rate when the order was originally purchased)