Using index 0 to insert milk and creamer into shopping_list_3.py

The progam as written assumes the user would enter a 1-indexed value, but doesn't check if a value is zero or negative. Python let's you use negative indexes to mean "from the end of the list". So when you enter a "0", the program inserts two items. The first at index -1 (that's your zero - 1), then the second item by incrementing the index from -1 to 0. Looking at an example:

Given a list ["a", "b", "c", "d"], insert two items "m1, z" at "position 0". Since the program substracts 1 from the starting position, the starting index will be -1:

# Make a list
In [19]: the_list = ["a", "b", "c", "d"]
# Insert first item "m1" at index -1
In [20]: the_list.insert(-1, "m1")
# review list
In [21]: the_list
Out[21]: ['a', 'b', 'c', 'm1', 'd']
# Insert second item "z" at index 0 (-1 + 1)
In [22]: the_list.insert(0, "z")
# review list
In [23]: the_list
Out[23]: ['z', 'a', 'b', 'c', 'm1', 'd']

In your case, milk is inserted at index -1 and creamer is inserted at index '0'.