Using Magento 1.9. I was not able to find an extension that does this as far as i can tell, and i cannot set this up using the promotion rules.
I want to setup 4 different fixed shipping rates, free, 50, 100, 150, but based on the largest item in my cart. These are based on product size. For example 4 different sizes light, small, medium and large, i setup all products to have this attribute and set which size they are. I would like the fixed rate shipping option to be based on the largest item in the cart. So if 2 small items and one large item are in the cart the fixed rate is still only 150. If only light items are in the cart you get free shipping.
I hope that makes sense.