Shipping carriers now use dimensional weights when calculating shipping rates. Depending on the size of your order, DIM (dimensional) rates may apply.
So what is DIM weight?
Basically it is a way for shipping companies to charge based on the amount of space a package takes and not just the weight. By charging only the weight of a package, lightweight packages become unprofitable for shipping carriers due to the amount of space they take up in proportion to their actual weight. So if a package is over a certain size, they use the length, width, and height of package to determine the dimensional weight. If the actual weight of the package is more than the DIM weight, they will use the higher weight to calculate the rate.
Confusing...it can be. To give an example, our Large and Medium Tobacco Lath Ladders can require dimensional shipping. FedEx uses DIM weight no matter where you are located. USPS only uses DIM (for our rates) if you are located farther from us (Pennsylvania).
For more on Dimensional weights here are a couple links to the carrier specific information.