The sales price on which the tax is due is the total amount received by the seller for the transaction. If the coupon is one for which the seller is reimbursed by a third party, such as a manufacturer’s coupon, the sales tax is due on the sales price before the coupon is applied. In this situation, the seller would still receive the full selling price for the transaction – partly from the consumer and partly from the third party.
If the coupon is a dealer discount coupon for which the seller is not reimbursed by a third party, the amount the seller actually receives is what is paid by the customer. In this situation, the seller would collect sales tax on the sales price after the coupon is applied.