From Brotato Wiki
Item
Coupon is an in-game item that can be bought from a Shop or can be found in a Crate. Most Items modify the Stats of the player, and some of them provide additional effects.
Coupon
Coupon reduces the Items Price secondary stat by 5%. This is an additive modification.
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.
Since the modification is additive, the economic value (= possible return) of each Coupon is identical: it lowers the price for an item by 5% of its base price. This is true independent of the current % Items Price modification for the character.
The highest cost reduction possible is 50%, on the character 
Entrepreneur and having the maximum amount of 5 coupons.
Strategy
The Coupon is an efficient economic item that provides a net gain in most runs, unless bought very late (outside of endless mode).
Since the base price of the Coupon itself is low, so is the opportunity cost of buying it, and it does become a net positive investment rather quickly. This means that Coupons should usually always be purchased if offered.
Late game Coupons will often not be able to recoup the initial investment, and should therefore be skipped (or recycled, if found in a loot crate, depending on the recycle value).
Since the Items Price stat is modified additively by the Coupon, the effectiveness of the item does not change for characters that start with a modifier to this stat (such as Entrepreneur who starts at -25% Items Price, or Saver who starts at +50% Items Price). However, for characters with an increased items price, the initial investment for the Coupon itself is also higher, which needs to be taken into account.
Coupons are very strong in endless mode, where they continue to provide their benefits past wave 20.