$ python3 inventory.py

1) Load product inventory from file.
2) Display individual product details.
3) Display all product details.
4) Add product to inventory.
5) Build invoice.
0) Quit.
What would you like to do? -1
Please enter an integer between 0 and 5.
What would you like to do? 10
Please enter an integer between 0 and 5.
What would you like to do? 0
Bye!