Add Special Shipping Costs to Magento Store

We need to add an additional shipping cost to orders. Every 20 lbs of product in the shopping cart should add $9 in what we currently refer to as "additional cost". This covers the shipping cost of the dry ice as well as the cost for the insulated cooler. A shipment of 1-20 lbs should see a $9 charge. Displayed as a separate line item as when making a purchase on the current site. 21- 40 lbs sees an additional charge of $18, and so on. This is in addition to the UPS API shipping charges.