At Cratejoy we get our shipping rates directly from USPS. The shipping rates that your customers are charged are essentially the same as those that can be found at the USPS Postage Price Calculator.
When using the calculator make sure to select that the items is a Package and enter the weight.
For packages that are less than a pound, Cratejoy shipping calculates using the First-Class Mail Parcel cost. For packages greater than a pound, the Standard Post cost is used.
Merchants obtaining postage through Cratejoy don't need to take any additional action to take advantage of cubic shipping rates. When the origin, destination, weight and box dimensions are entered, the cheaper rate of weight vs cubic is automatically calculated for purchasing postage.