validating user input and loops

[nilamo]

    mon = input("How many bottles did you return on Monday: ")
    is_valid = is_valid_integer(input_string)
    while not is_valid:
        mon = input("Nice try, please enter a whole number! ")
        is_valid = is_valid_integer(input_string)

You're checking if input_string is a number, but you never set input_string, so it'll never be a number.  Instead, you set mon, but then never use mon for anything.

That's why you're stuck in the loop.