The shock cord, or bungee cord, is used for the pocket flaps on the Randonneur
I have three colors of shock cord, which all use reflective tracers.
If you are riding at night and use a headlamp or secondary light, when you look down to open the bag, the light will pick up the reflective tracers.
This will make accessing the bag at night a little bit easier, especially when you are tired and it is difficult to focus.
Additionally, the reflective tracers offer a little bit of visibility to oncoming car traffic.
There is no additional charge for custom shock cord options.