We do support a scenario for adjusting shipping cost by a unit such as weight. Navigate to the Product Details page for your product and choose Edit Price & Shipping from the menu on the right. Click Next twice to move to the Shipping Units page. On this page you can enter a number that will multiply the per item cost of shipping that item. For example, for a five-pack of socks you may wish to enter 5.00 in that box, so that the per-item shipping cost will be the same as five individual pairs of socks.