Dim weight is a reality as you start selling larger items or combining multiple sales in to the same package. It is a weight that is calculated based upon the size of the box the item ships in. If the dimensions say 20 pounds but your scale says 2 pounds, you are being charged by the dimensional weight and not the actual weight. Knowing how this plays out will make sure you are charging enough to cover the cost of shipping. Remember, whether you charge for your postage or not, you absolutely have to factor that in to your sale price. More to come on this, let me know what you see as a challenge for your business.