Oscommerce Individual Shipping Custom Calculation

Hi, We use Individual Product Shipping costs on our oscommerce website. I need a tweak doing so that when a customer orders multiple products only the Highest and Lowest shipping costs are combined. So, if a customer orders 5 products each with a different shipping fee, the lowest shipping is added to the highest to create a total shipping cost - the 3 in the middle are not included. Any questions just ask. This is urgent and must be done within 24 hours. PLEASE READ CAREFULLY There is NO access to the website. I can provide all the files you need, but live access to the actual website is not possible for security reasons. Payment in full upon completion. As long as I can see it working I'm happy. Thanks