Let's say they are all in categories_id 10 and use the master_categories_id 10 ...
You could calculate a discount on these in the perweightunit.php
Code:
global $cart;
$chk_cat_10 = $_SESSION['cart']->in_cart_check('master_categories_id','10');
$chk_discount = (($chk_cat_10-1) * 48.75) * .10;
echo 'I see Cat 10: ' . $chk_cat_10 . ' discount: ' . $chk_discount;
$total_weight_units = $shipping_weight;
$this->quotes = array('id' => $this->code,
'module' => MODULE_SHIPPING_PERWEIGHTUNIT_TEXT_TITLE,
'methods' => array(array('id' => $this->code,
'title' => MODULE_SHIPPING_PERWEIGHTUNIT_TEXT_WAY,
'cost' => MODULE_SHIPPING_PERWEIGHTUNIT_COST * ($total_weight_units * $shipping_num_boxes) - $chk_discount +
(MODULE_SHIPPING_PERWEIGHTUNIT_HANDLING_METHOD == 'Box' ? MODULE_SHIPPING_PERWEIGHTUNIT_HANDLING * $shipping_num_boxes : MODULE_SHIPPING_PERWEIGHTUNIT_HANDLING) ) ));
The echo can be commented out later this is just to show you how many are found and the discount ...
You can adjust how the discount is made etc.
You can add in more categories to check or combine the count from more categories before doing the discount or what ever you like ...