I s it possible to base the total on a weight table rather than setting a price for each item? My client sells products that all have the same price per unit (lb). He would like to have

0 to 10 lbs at $5 per pound
>10 to <20 at $6 per pound
20 pounds at $5 per pound
>20 to <30 at $5 per pound
30 at $5
>30 to <40 at 5$ per pound and so on.

I figured we would simply set up an option for selecting how many pounds for each product and set the price to 0, then apply the calculation at checkout using the total weight and the table logic above to calculate the total price. There will no shipping or taxes charged. My question is, which file and where in it is the order total calculated?

Thanks for this great forum!