Referencing Combo Box

[MC2020]

Hi - I am trying to write a Python code to basically do a sumproduct function based on the item selected and the quantity of that item selected.

My code is below. I am having trouble referencing the combobox values. The calculate function is where I'm going wrong. How do I reference the comboboxes I have inputted onto the 'NewWindow'? I add comboboxes to the page based on the number of items selected and the all have the same values,etc.

For example if I select 2 'pizzas' and 1 'CocaCola' then the code should print 33. ((2*$15)+(1*$3))

This is my error:

Error:
    cost = fooditems2[x] * combobox.get()
TypeError: can't multiply sequence by non-int of type 'str'

fooditems = {'pizza' : [15] , 'breadsticks' : [5] ,'wings' : [10],'CocaCola' : [3] ,'brownie' : [2]}
fooditems2 = []
quantity = ['1','2','3','4']



import tkinter as tk
from tkinter import *
from tkinter import ttk

menu = Tk()
menu.geometry('500x300')

check_boxes = {item:tk.IntVar() for item in fooditems}

for item in fooditems:
    cb = tk.Checkbutton(menu, text=item, variable=check_boxes[item], anchor='w', onvalue=1, offvalue=0, width=50)
    cb.pack()

combobox = ttk.Combobox(menu, values=quantity)


def Open():
    New_Window = Toplevel(menu)
    New_Window.geometry('500x300')
    calculateButton = tk.Button(New_Window, text = 'calculate', command=calculate)
    calculateButton.place(x=250,y=250)
    for key, item in check_boxes.items():
        if item.get() == 1:
            fooditems2.append(key)
    for x in range(len(fooditems2)):
        b = Label(New_Window, text=fooditems2[x])
        b.pack()
        combobox = ttk.Combobox(New_Window, values=quantity)
        combobox.pack()
    New_Window.mainloop()

def calculate():
    for x in range(len(fooditems2)):
        cost = fooditems2[x] * combobox.get()
        print(cost)

confirmButton = tk.Button(menu, text = 'Confirm', command=Open)
confirmButton.place(x=250,y=250)



menu.mainloop()