Adapt shipping code for Shopify website

Hello, We use the Fedex, USPS, and UPS APIs to calculate shipping costs for products on our Shopify-based website. We have one type of product that ships in two 66-pound boxes. Our problem is that the shipping rate calculator for this product returns the rate for shipping 132 pounds, when the way we actually ship it is in two 66-lb boxes, and the rate for shipping 132 lbs is significantly higher than the rate for shipping 66 lbs, times two. What we would like is for the shipping rate calculator to recognize when this product is in the shopping cart, and return the 66-lb shipping cost times two, rather than the 132 lb shipping cost.