Bonfire Sale is a power up that gets dropped. I believe it is only in "FIVE".

The icon looks like a Pack-a-Punch machine. When you get it, all of the teleporters except the one in the PaP room (usually called Safe Room) will take you to the PaP room and the PaP costs a mere 1000 points instead of the usual 5000.

I have gotten the occasional Bonfire Sale at random, but usually it comes from killing the Zombie Thief (a.k.a. Zombie Scientist) and then it is usually from killing him before he steals any guns.

I have gotten it at about 30% of the times that I have killed him before he took any weapons, so it isn't guaranteed that you will get one.