Tax authority rules typically require that money-off coupons be
applied as cash would. For example, if you had an order which
contained two items of similar price, one of which had tax and the
other that did not, if you applied the coupon to the taxable one to
reduce its price, you thereby reduce or even zero the tax due on the
order. However, if you applied the coupon to the non-taxable item, the
total tax on the order remains the same. Similarly, coupons must
always be applied to the order total after tax, just like cash would.
The value of a coupon should be evenly applied across the tax
inclusive prices of all items in the order in proportion to their
value. Kartris does this, and will also calculate the % split of the
coupon itself between the tax and order value.