After you have created the Shipping Rules, make sure that they are in the correct order.
Most merchants will prefer to organize this set with the rules assigning the greatest DIM weight packaging first, and the smallest DIM weight packaging last. This is because the Shipping Rules are applied to your orders from top to bottom, as displayed on the SHIPPING RULES SETUP page. The rules lower on the list will override the ones up top - and you want to be sure to use the smallest package possible.
Arrange your Shipping Rules by clicking the arrow direction for each rule and place them in the order that you want them to be applied.