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.