Yes, I feel both your math and use of the stamps is correct. I have one additional recommendation of writing the value of each stamp alongside (or above or below) each stamp. I did something similar recently. It was returned due to insufficient potage, as a USPS employee processing the envelope believed nonmachineable surcharge stamps were valued at 55¢. The next day the window clerk affirmed that my math was correct and apologized for the hassle.
In regards to how you calculated the fee at $3.20, I also believe you are again correct unless it is rigid, non-rectangular, or contains thickness fluctuations in excess of 1/4-inch.
Please remember that items weighing 10 ounces or more must be presented to a window clerk at a retail counter, unless postage is paid (at least partially) via credit card via a kiosk, etc.