This is called a "Conical Stop for Roll Up Shutters" and comes in packs of 6 , 12 or 20 units. A package of 6 is $11.
This little part can save hundreds of dollars in maintenance and repair costs. How? 
It stops the shutter when it is rolling up, so that it doesn't completely go inside the box, unable to come out again until handled by the repair guy.
 
Remember, when you buy more you pay less.