Question re: coupons

We have a customer who has been doing coupons as a tender type but recently realized that this is not the correct way to do it for tax reasons.
Example: if a customer buys an item that is $100 and there is a tax of 5%, then the total sale is $105 which is normal. If the customer has a $20 coup on, he still owes $85 which is incorrect. The tax has to be taken on the di scounted amount, so it would be $100 - $20 = $80 + 5% = $84.
We were going to set up a non-inventory item with an open price to accommod ate all possible coupon values, but this introduces the possibility of user error if they forget to include the minus sign. We could also have them do a line item discount or modify the price of the item but again, these all introduce extra steps and extra opportunities for mistakes.
Is there an easier way to do this?
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