11.35 is the cost for a medium flat rate box.
Since you are using the flat rate box service, it is not using weight, the flat rate box service does not factor weight, "If it Fits, it ships" or so they say.
if you wanted some items to use the medium flat rate box, and others to use the large flat rate box. You would need to add a column to your cart for package size, and in the add to cart button behavior, set that column to "LARGE" or "REGULAR". then in the USPS Behavior, bind the package size to the cart column