Bottom Attachment System: Per our discussion, I've made a a heavy duty attachment system to attach to the bag of the rack, at the widest point. To take the bag off, your courier will have to open a left flap, a bottom strap, and a right side flap that all velcro "sandwiches" together. There are also a front, left, and right bottom straps that secure to the bottom front and side panels of the bag. After putting the bag onto a bike, I think that we should add an additional left and right strap to reduce the minimal shifting that may occur.

See photos and Video, Also, Please note, that the stiff white plastic panel in the photos below is the actual stiffener that is inserted into the bottom of the bag via an exterior flush pocket.