Just checking out this Lemmy thing. Also, on Mastodon.
It is not legal to charge a different price at the register. Also, it would be quite difficult to ensure the price you pay at the register matches the price you saw on the shelf ten minutes ago.
You can start with Tkinter since it’s a standard python library. Here’s a tutorial to get you started.
https://www.geeksforgeeks.org/python-tkinter-tutorial/