Just so I can be completely clear and clean in my coding, I have added the following to my Add to Cart "form" element
<input type="hidden" name="TotalPrice" value="<?php
$x = $row_rsDetails['price'];
$y = $row_rsDetails['ShippingCubes'] * $row_rsDetails['CostperCube'];
$a = $row_rsPackageShipping['ShipSum'];
$b = $y + $a;
echo bcadd($x, $b, 2);
?>" />
and in the header within the Add to Cart object, I have added this coding
$ATC_itemPrice = "".((isset($_POST["TotalPrice"]))?$_POST["TotalPrice"]:"") ."";// column binding
Is this all I need to do? Does this look correct as far as you can tell?