About coupons and taxes
When using coupons in WooCommerce, it's important to understand how coupons are applied and their impact on tax calculations in your shop.
Taxes are calculated on a per-product basis. When adding a coupon to a product or cart, the coupon amount will be applied to the individual product price before tax is calculated. This allows the discount to be split between all items in the cart for accurate tax calculations.
To prevent rounding issues, WooCommerce will not split cent values. When a discount is split between multiple items in the cart and the split is not even (like a $10 fixed cart discount split between three items), then the remaining discount amount is applied by line item until used up. The final total discount will equal the coupon amount. For example, a $10 fixed cart discount applied to three items would be split as $3.33, $3.33, and $3.34.