You're not assigning the parsed float back to your value var:

value = parseFloat(value).toFixed(2);

should fix things up.

toFixed isn't a method of non-numeric variable types. In other words, Low and High can't be fixed because when you get the value of something in Javascript, it automatically is set to a string type. Using parseFloat() (or parseInt() with a radix, if it's an integer) will allow you to convert different variable types to numbers which will enable the toFixed() function to work.

var Low  = parseFloat($SliderValFrom.val()),
    High = parseFloat($SliderValTo.val());

toFixed() method formats a number. The current value is of type string and instead of arithmetic addition, string concatenation is happening. Convert those to number before adding:

Change:

var totalSum = (grandTotal + getShippingCost).toFixed(3);

To

var totalSum = (Number(grandTotal) + Number(getShippingCost)).toFixed(3);

I think the easiest way to prevent the error from happening is to always parse the parameter as number:

var total = 0;
function add(a){
  total+=a;
  var float_num = Number(total).toFixed(2);
  return float_num;
}