Generally, customers buy discount vouchers and group coupons online and then bring them to the retailer for the discount. The purchase of a discount is not taxable. If the item (or service) is taxable, charge sales tax when the voucher or coupon is redeemed.
Do you know how much the customer paid for the voucher?
Charge sales tax on:
Amount paid for the discount voucher
Face value of the discount voucher
Note: Your records must support the tax calculation.