To accomplish the first part of charging $30 for 6lbs of one and 3lbs of another, you could add a product that costs $30 and then have 2 simple options to get the 6lb choice and the 3lb choice.
If you have eCart, you can edit the eCart object to add a discount to it. You would want to make it based on the Number of items with a specific column value and then check the ID for the ID that the product you added is and then set up the discount there.