Also, they count as your carry on bag, and they only bring one parachute on board, so if multiple people have to be thrown off and they've both paid for a parachute, they have to draw straws to decide who gets it.

Edit: And remember, OOF goes off on less than .1% of flights, so they have hundreds of times as many parachutes per flight as they have people who need them. Rumors that parachutes are oversubscribed are therefore wildly inaccurate.