Would it work if you could setup for Total >= 75.00 and weight <= XX or do you need to make sure specific Products are not part of the weight that makes up the XX ...

On the Products that are heavy, is there something in the products table that distinguishes them from the light weight Products such as the master_categories_id ...

If not, you would need to customize the products table to make the Heavy Products so you can check if they are in the Order ...

If there is 1 heavy weight Product in the cart, what should happen?

If there are 3 Light weight Products in the cart and the order is >= 75 what should happen?

If there are 27 Light weight Products in the cart and the order is >= 75 what should happen?

If there is 1 heavy weight Product and 5 Light Weight Products in the cart and the amount is >= 75.00 what should happen?