I have figured out the design and layout of my app, yet I have a bug trying to...

I have figured out the design and layout of my app, yet I have a bug trying to get the extra charge of bacon to the final price. I think it needs me to call the function for the checkbox for bacon, but I am not positive.

main.py

 from kivy.app import App
-from kivy.modules import inspector # For inspection.
-from kivy.core.window import Window # For inspection.
+from kivy.modules import inspector  # For inspection.
+from kivy.core.window import Window  # For inspection.
+from kivy.properties import BooleanProperty, NumericProperty
 
 __app_package__ = 'edu.unl.cse.soft161.order'
 __app__ = 'Order Meal'
@@ -9,8 +10,19 @@ __flags__ = ['--bootstrap=sdl2', '--requirements=python2,kivy', '--orientation=l
 
 
 class OrderApp(App):
+    original_price = NumericProperty(12.99)
+    bacon = BooleanProperty(False)
+    extra = NumericProperty(2.99)
+    total = NumericProperty(12.99)
+
     def build(self):
-        inspector.create_inspector(Window, self) # For inspection (press control-e to toggle).
+        inspector.create_inspector(Window, self)  # For inspection (press control-e to toggle).
+
+    def update_total_charge(self):
+        if self.root.id.bacon(False):
+            self.total = self.original_price
+        elif self.root.id.bacon(True):
+            self.total = self.original_price + self.self.extra
 
 
 if __name__ == '__main__':

order.kv

 BoxLayout:
     orientation: 'vertical'
     Label:
-        text: '[Your meal-ordering GUI here]'
+        text: 'Steak-Stop'
+        font_size: '40sp'
+        size_hint: (1,.1)
+    BoxLayout:
+        orientation: 'vertical'
+        BoxLayout:
+            size_hint: (1,.3)
+            Label:
+                text: 'Menu'
+        BoxLayout:
+            orientation: 'vertical'
+            Button:
+                text: 'Rib-eye'
+                size_hint: (1,.1)
+            Button:
+                text: 'T-Bone'
+                size_hint: (1,.1)
+            Button:
+                text: 'Fillet'
+                size_hint: (1,.1)
+            Button:
+                text: 'Prime rump'
+                size_hint: (1,.1)
+            Button:
+                text: 'flat-iron'
+                size_hint: (1,.1)
+        BoxLayout:
+            orientation: 'horizontal'
+            Label:
+                text: 'rare'
+                font_size: '32sp'
+            Slider:
+                id: cook_time
+                range:(0,100)
+                value: 50
+            Label:
+                text: 'Well-done'
+                font_size: '32sp'
+        BoxLayout:
+            orientation: 'horizontal'
+            CheckBox:
+                font_size: '25sp'
+            CheckBox:
+                id: bacon
+                font_size: '25sp'
+            CheckBox:
+                font_size: '25sp'
+            CheckBox:
+                font_size: '25sp'
+        BoxLayout:
+            orientation: 'horizontal'
+            Label:
+                text: 'steak sauce'
+                font_size: '15sp'
+            Label:
+                text: 'Bacon strip around steak'
+                font_size: '15sp'
+            Label:
+                text: 'Barbeque sauce'
+                font_size: '15sp'
+            Label:
+                text: 'peppercorn sauce'
+                font_size: '15sp'
+        BoxLayout:
+            orientation: 'vertical'
+            CheckBox:
+                text: 'idk'
+                font_size: '25sp'
+            Label:
+                text: 'Steak price' + str(app.total)
+                size_hint: (.25,1)
+                font_size: '20sp'