Generally, we quote shipping cost based on the product dimensions for each item in an order. This is based on the dimensions that are provided in the product catalog.
The dimensional weight calculation is usually (length x width x height)/139. If the result of this calculation is higher than the actual weight in pounds, then we charge shipping based on the dimensional weight. If it ends up being less, we charge based on the actual weight of the item.
We use the product catalog dimensions because on checkout we do not have information available for what size boxes we will be receiving in house, so the catalog is the only information available to us.
For more information on this please review this article.