Should my coupon come off before or after tax?

It changes the amount tax is worked out on:

• Before tax (tax on discounted amount): the discount comes off first, then tax is charged on the lower amount. The customer pays less tax.
• After tax (tax on full amount): tax is charged on the full price, then the discount comes off.

Set it in Marketing → Coupons when you create or edit a coupon, under Discount & tax, then click Save coupon. It doesn't apply to Free shipping coupons. If you're unsure which your tax rules require, ask your accountant.