What you are talking about is not allowed for work done under Apple.
The top case is one part which cannot be disassembled so far as they are concerned. It contains the main part of the shell, the keyboard, the trackpad, and the battery. A failure in any part of it is to be serviced by a complete replacement of the top case.
It is not an exceptionally expensive component, but it is a significant amount of labor. The top case is pretty much the "base" that all other components are attached to. Which means you take everything else out (logic board, display, fans, vent, etc) and move it to the new one.
Also, the touch ID bit is a bitch to move.