I think you are talking about the A.R.E. cap which we had installed at a local shop. We went with this type of cap so we could have easier access to the back without the tailgate, it makes loading and unloading much easier and we can open the door if we just want to grab something quick. The windows open for easy access to gear up by the cab and the liftgate opens up or thru the door. It's a couple hundred pounds, I'm not sure we'll remove it as we never did on our other truck. Got a good price as we were repeat customers and we were getting step tubes from him and a few other mods.
If you're talking about the boxes inside, they were part of a GMC business owner program so they didn't cost anything and are attached to the inside rail system that we had put on when we ordered the truck. The side boxes are nice as you can remove them by undoing the latch. They all lock. Hope that was what you wanted to know.

Well Hoorah... Where did you get that great setup? I have a Silverado with the moveable box,
but just a tonneau cover. Is yours secure? Heavy? Expensive?