If all products except the skateboards are charged based on USPS charges, then you could set the skateboard weight to 0 to not affect that charge and then count the number of skateboards to add that charge to the USPS charge ...
All that is needed is something to identify the skateboards, such as 0 weight on them using:
Code:
$skateboards = $_SESSION['cart']->in_cart_check('products_weight', '0');
This would return the item count of the skateboards based on those Products using a products_weight of 0 ...
So if 1 is in the cart you get 1, if 3 are in the cart you get 3 ... now you can determine how much more to add to the USPS charge for the other products ...