From Brotato Wiki
Coupon reduces the Items Price secondary stat by 5%.
It is the only item in the game that modifies shop prices.
Shop price reductions stack additively, and the current total can be seen in the secondary stats tab, as % Items Price. You can stack up to 5 Coupons for a maximum 25% price reduction. (Items will cost 75% of the default price.
The highest cost reduction possible is 50%, on the character Entrepreneur and having the maximum amount of 5 coupons.
The Coupon is an efficient economic item that provides a sizeable net gain in most runs, unless bought very late (outside of endless mode). It is generally considered one of the better items in the game, and is nearly always bought in shop when seen, unless it is found at wave 16-18 or later.