1. Make it look visually nice so cashiers will not suspect anything is wrong....which requires a graphics expert. Dollars and other currencies have special security features. Coupons do not have a way to detect forgeries.

2. All UPC codes, credit cards, and general codes have check digits. Companies uses a different algorithms for the check digit. Some add the digits, and see if the final UPC digit is the last digit of the sum of all other digits. There are more complicated algorithms, but keeping it private is essential. Especially for dynamically created coupon codes. I can make coupons for 99% OFF, as long as the check digit matches. Keep in mind that coupons are not currencies, hence it is nearly impossible to find a fraud in a single instance. The authorities can only catch people that go too far overboard, and repeatedly perpetuate the fraud.