Shipping costs associated with a dropdown selection

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I sell coffee beans by the pound at [url]www.thebeanstock.com[/url] and have a dilemma that I hope you can help me with.  

I have listings for 1 pound, 2, 5, and bulk (20 lbs) for each varietal (ie. Colombia Supremo). The more beans you buy the cheaper the "per pound" price.

Using Col Supremo as an example, currently I have to list each Colombia Supremo four different times with separate prices and the actual weights (please recall that when listing a product we indicate the weight so the shipping calculator knows how much to calculate).  What I want to be able to do is list Colombia Supremo just once and offer a drop down selection menu showing 1,2,5, and bulk(20) with their associated discounted pricing. (the more you buy the cheaper per pound) 

I know how to do all that..... but here is the dilemma for what I want to do:

[b]Lets say you are a customer in my Colombia Supremo listing and wish to buy 5 pounds so you click on the drop down and choose the 5 lbs. option.  (Here I can add the price or dollar increase for the 5 pounds and understand all that), but what tells the shipping calculator to add 5 pounds to the shipping costs?  In fact, if my drop down has a value of 1, 2, 5, and 20 pounds how do we acknowledge the weight? [/b] 

Put very simply to clarify:  If I were selling widgets and the optional addon item in the dropdown was 25 pounds or 100 pounds? I can add an additional item fee, but now how do I get the shipping calculator to add 25 or 100 pounds depending on the choice?

Can you help me?  :cry: