Upon use, turns a random item in the current room that costs money or hearts into a pedestal item that can be taken for free.
While held, guarantees a random item or pickup from the Shop to be half price.
Notes
Coupon has a 1% chance to drop from destroying normal Shopkeepers.
Using Coupon in a Devil Room and taking that free item does not affect or remove the chances for Angel Rooms.
Using Coupon in Greed mode does not restock the taken item or consumable.
Using Coupon in a Shop while holding Restock will not respawn a new item after taking the free one.
Using Coupon as The Lost or Tainted Lost allows multiple Devil deals to be taken.
Synergies
Book of Virtues: Spawns green wisps on the middle ring. The wisps fire three tears at a time, similarly to The Inner Eye. However, all of these wisps are destroyed if Isaac buys something using money. Does not affect wisps other than ones made by this item.
Shop to be half price.
Shopkeepers.
Devil Room and taking that free item does not affect or remove the chances for 
Angel Rooms.
Restock will not respawn a new item after taking the free one.
Using Coupon as 
The Lost or 
Tainted Lost allows multiple Devil deals to be taken.
Book of Virtues: Spawns green wisps on the middle ring. The wisps fire three tears at a time, similarly to 
The Inner Eye. However, all of these wisps are destroyed if Isaac buys something using money. Does not affect wisps other than ones made by this item.
Car Battery: Makes two items free upon use.
Satanic Bible: The Devil deal in the boss room can be acquired for free.
Steam Sale: The discounts stack.
