Different shipping methods for different categories and mixed categories

After reading through other threads I am I not much clearer on what to do
some of the others issues are similar to mine but there has been no clear instructions.

I plan on selling 2 products on my website currently we are on selling higher value products:

1. Higher value products (category a)
2. Lower value products (category b)
3. Other categories combination of products in category a and b)

I currently have the clat rate shipping module cloned 8 times with different zones set to each clone.

My shipping prices are fixed for each zone no matter if its a higher value product or a lower value product I still get charged
the same from the courier.

For 2 of these cloned flat rate zones I offer free shipping on higher value products and we absorb the cost of the courier.

For the zones with free shipping we are unable to absorb the courier charges if a customer purchases a lower value product
only as we would then be selling the product at a loss.

So what I am trying to setup is a shipping modification
that will:

a) higher value product = shipping free
b) higher value and lower value product = shipping free
c) lower value product = shipping rate £5

Having read through the thread the nearest I have found to
what I am looking to do is this but I am not sure on this and
what code to edit.

// disable for one master_categories_id
      if ((IS_ADMIN_FLAG == false && $_SESSION['cart']->in_cart_check('master_categories_id','17') > 0)) {
          $this->enabled = false;
      }  

I guess no one can help with this query.

You have the right idea ...

To check how many products are in the shopping cart from master_categories_id 17 and disable if greater than 0, then you need to use:

// disable for one master_categories_id
      global $cart;
      if ((IS_ADMIN_FLAG == false && $_SESSION['cart']->in_cart_check('master_categories_id','17') > 0)) {
          $this->enabled = false;
      }  

Most likely the missing:
global $cart;

is what is causing an issue ...

global $cart will not trip the error in the Admin but the test of the $_SESSION['cart'] would ...

Using the IS_ADMIN_FLAG && in the IF will prevent the error for the test of the cart, as you have it setup ...