Python checkbutton variable issue

I've written this code to get the variable from a checkbutton across two separate windows, as shown below...

However, the checkbutton variable doesn't update, and every time the checkbutton is clicked, the program will always output 'variable is 0.'

from tkinter import *
from tkinter import ttk

e9 = None   # extras checkbox
checke9 = 0      # variable to check value of extras checkbox


def checkcb() :
    global checke9
    print("variable is " + str(checke9.get()))

def customer_window() :

    # assign global variables
    global checke9
    global callroot
    global e9

    # assign variable for the extras checkbox
    checke9 = IntVar()  
    checke9.set(0)

    # close previous window
    callroot.destroy()


    # set customer info
    customer = Tk()
    customer.resizable(width=False, height=False)
    customer.configure(bg = "#c8ebf7")

    # create extra choices
    Label(customer, text="Checkbox: ", anchor="e", font="Corbel 12 italic", width="12", bg="#c8ebf7").grid(row=9)
    e9 = Checkbutton(customer, text="Click here...", variable=checke9, offvalue=0, onvalue=1, font="Corbel 12", bg="#c8ebf7", command=checkcb)
    e9.grid(row=9, column=1, sticky=W)

    customer.mainloop()




# define function to open the main (root) window
def root_window() :

    # set root info
    root = Tk()
    root.resizable(width=False, height=False)
    root.configure(bg = "#c8ebf7")

    global callroot
    callroot = root

    # create title label
    Button(root, text="Open Window...", font="Corbel 14 bold", width=20, command=customer_window).grid(row=1, column=0, pady=30)
    Button(root, text="Close", font="Corbel 8", width=10, command=root.destroy).grid(row=3, column=0, pady=5)
    root.mainloop()



# start program
root_window()

Can you help? Thanks in advance.