I am having the same problem. Turns out it's a bug with "Shop Tile Framework" not properly supporting Gamepad Mode. In my case I still cannot interact with the register with a gamepad, but if I click on the register with my mouse it will work. I found a workaround for gamepad only though: On my PS4 controller I move the right joystick to move the mouse pointer over the register then press the X button. Doing so allows me to use the register fully with the controller.
I hope this helps!
P.S. - I actually found this thread while googling the issue when I was trying to figure out what to do. I know the thread is a few months old, but since it has no responses I figure providing a solution would be good even if the OP may have figured it out by now.
